Enable `Performance/UnfreezeString` cop

In Ruby 2.3 or later, `String#+@` is available and `+@` is faster than `dup`.

```ruby
# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips"
end

Benchmark.ips do |x|
  x.report('+@') { +"" }
  x.report('dup') { "".dup }
  x.compare!
end
```

```
$ ruby -v benchmark.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
Warming up --------------------------------------
                  +@   282.289k i/100ms
                 dup   187.638k i/100ms
Calculating -------------------------------------
                  +@      6.775M (± 3.6%) i/s -     33.875M in   5.006253s
                 dup      3.320M (± 2.2%) i/s -     16.700M in   5.032125s

Comparison:
                  +@:  6775299.3 i/s
                 dup:  3320400.7 i/s - 2.04x  slower

```

8 files changed, 19 insertions, 19 deletions

diff --git a/activerecord/test/cases/adapter_test.rb b/activerecord/test/cases/adapter_test.rb
index 217ba22294..64c2b51f83 100644
--- a/activerecord/test/cases/adapter_test.rb
+++ b/activerecord/test/cases/adapter_test.rb
@@ -290,7 +290,7 @@ module ActiveRecord
       def test_log_invalid_encoding
         error = assert_raises RuntimeError do
           @connection.send :log, "SELECT 'ы' FROM DUAL" do
-            raise "ы".dup.force_encoding(Encoding::ASCII_8BIT)
+            raise (+"ы").force_encoding(Encoding::ASCII_8BIT)
           end
         end
 
diff --git a/activerecord/test/cases/adapters/postgresql/bytea_test.rb b/activerecord/test/cases/adapters/postgresql/bytea_test.rb
index 64bb6906cd..3988c2adca 100644
--- a/activerecord/test/cases/adapters/postgresql/bytea_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/bytea_test.rb
@@ -49,7 +49,7 @@ class PostgresqlByteaTest < ActiveRecord::PostgreSQLTestCase
   end
 
   def test_type_cast_binary_value
-    data = "\u001F\x8B".dup.force_encoding("BINARY")
+    data = (+"\u001F\x8B").force_encoding("BINARY")
     assert_equal(data, @type.deserialize(data))
   end
 
diff --git a/activerecord/test/cases/adapters/postgresql/money_test.rb b/activerecord/test/cases/adapters/postgresql/money_test.rb
index 61e75e772d..75e5aaed53 100644
--- a/activerecord/test/cases/adapters/postgresql/money_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/money_test.rb
@@ -52,10 +52,10 @@ class PostgresqlMoneyTest < ActiveRecord::PostgreSQLTestCase
 
   def test_money_type_cast
     type = PostgresqlMoney.type_for_attribute("wealth")
-    assert_equal(12345678.12, type.cast("$12,345,678.12".dup))
-    assert_equal(12345678.12, type.cast("$12.345.678,12".dup))
-    assert_equal(-1.15, type.cast("-$1.15".dup))
-    assert_equal(-2.25, type.cast("($2.25)".dup))
+    assert_equal(12345678.12, type.cast(+"$12,345,678.12"))
+    assert_equal(12345678.12, type.cast(+"$12.345.678,12"))
+    assert_equal(-1.15, type.cast(+"-$1.15"))
+    assert_equal(-2.25, type.cast(+"($2.25)"))
   end
 
   def test_schema_dumping
@@ -65,7 +65,7 @@ class PostgresqlMoneyTest < ActiveRecord::PostgreSQLTestCase
   end
 
   def test_create_and_update_money
-    money = PostgresqlMoney.create(wealth: "987.65".dup)
+    money = PostgresqlMoney.create(wealth: +"987.65")
     assert_equal 987.65, money.wealth
 
     new_value = BigDecimal("123.45")
diff --git a/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb b/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb
index 7b73621ed9..70d0a71c53 100644
--- a/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb
+++ b/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb
@@ -167,7 +167,7 @@ module ActiveRecord
             data binary
           )
         eosql
-        str = "\x80".dup.force_encoding("ASCII-8BIT")
+        str = (+"\x80").force_encoding("ASCII-8BIT")
         binary = DualEncoding.new name: "いただきます！", data: str
         binary.save!
         assert_equal str, binary.data
@@ -176,7 +176,7 @@ module ActiveRecord
       end
 
       def test_type_cast_should_not_mutate_encoding
-        name = "hello".dup.force_encoding(Encoding::ASCII_8BIT)
+        name = (+"hello").force_encoding(Encoding::ASCII_8BIT)
         Owner.create(name: name)
         assert_equal Encoding::ASCII_8BIT, name.encoding
       ensure
diff --git a/activerecord/test/cases/batches_test.rb b/activerecord/test/cases/batches_test.rb
index c8163901c6..da4569e38a 100644
--- a/activerecord/test/cases/batches_test.rb
+++ b/activerecord/test/cases/batches_test.rb
@@ -155,7 +155,7 @@ class EachTest < ActiveRecord::TestCase
   end
 
   def test_find_in_batches_should_not_use_records_after_yielding_them_in_case_original_array_is_modified
-    not_a_post = "not a post".dup
+    not_a_post = +"not a post"
     def not_a_post.id; end
     not_a_post.stub(:id, -> { raise StandardError.new("not_a_post had #id called on it") }) do
       assert_nothing_raised do
@@ -420,7 +420,7 @@ class EachTest < ActiveRecord::TestCase
   end
 
   def test_in_batches_should_not_use_records_after_yielding_them_in_case_original_array_is_modified
-    not_a_post = "not a post".dup
+    not_a_post = +"not a post"
     def not_a_post.id
       raise StandardError.new("not_a_post had #id called on it")
     end
diff --git a/activerecord/test/cases/binary_test.rb b/activerecord/test/cases/binary_test.rb
index d5376ece69..58abdb47f7 100644
--- a/activerecord/test/cases/binary_test.rb
+++ b/activerecord/test/cases/binary_test.rb
@@ -12,7 +12,7 @@ unless current_adapter?(:DB2Adapter)
     FIXTURES = %w(flowers.jpg example.log test.txt)
 
     def test_mixed_encoding
-      str = "\x80".dup
+      str = +"\x80"
       str.force_encoding("ASCII-8BIT")
 
       binary = Binary.new name: "いただきます！", data: str
diff --git a/activerecord/test/cases/core_test.rb b/activerecord/test/cases/core_test.rb
index 6e7ae2efb4..f7fbf3ee8a 100644
--- a/activerecord/test/cases/core_test.rb
+++ b/activerecord/test/cases/core_test.rb
@@ -36,7 +36,7 @@ class CoreTest < ActiveRecord::TestCase
 
   def test_pretty_print_new
     topic = Topic.new
-    actual = "".dup
+    actual = +""
     PP.pp(topic, StringIO.new(actual))
     expected = <<~PRETTY
       #<Topic:0xXXXXXX
@@ -65,7 +65,7 @@ class CoreTest < ActiveRecord::TestCase
 
   def test_pretty_print_persisted
     topic = topics(:first)
-    actual = "".dup
+    actual = +""
     PP.pp(topic, StringIO.new(actual))
     expected = <<~PRETTY
       #<Topic:0x\\w+
@@ -93,7 +93,7 @@ class CoreTest < ActiveRecord::TestCase
 
   def test_pretty_print_uninitialized
     topic = Topic.allocate
-    actual = "".dup
+    actual = +""
     PP.pp(topic, StringIO.new(actual))
     expected = "#<Topic:XXXXXX not initialized>\n"
     assert actual.start_with?(expected.split("XXXXXX").first)
@@ -106,7 +106,7 @@ class CoreTest < ActiveRecord::TestCase
         "inspecting topic"
       end
     end
-    actual = "".dup
+    actual = +""
     PP.pp(subtopic.new, StringIO.new(actual))
     assert_equal "inspecting topic\n", actual
   end
diff --git a/activerecord/test/cases/type/type_map_test.rb b/activerecord/test/cases/type/type_map_test.rb
index f3699c11a2..1ce515a90c 100644
--- a/activerecord/test/cases/type/type_map_test.rb
+++ b/activerecord/test/cases/type/type_map_test.rb
@@ -32,7 +32,7 @@ module ActiveRecord
       end
 
       def test_fuzzy_lookup
-        string = String.new
+        string = +""
         mapping = TypeMap.new
 
         mapping.register_type(/varchar/i, string)
@@ -41,7 +41,7 @@ module ActiveRecord
       end
 
       def test_aliasing_types
-        string = String.new
+        string = +""
         mapping = TypeMap.new
 
         mapping.register_type(/string/i, string)
@@ -73,7 +73,7 @@ module ActiveRecord
       end
 
       def test_register_proc
-        string = String.new
+        string = +""
         binary = Binary.new
         mapping = TypeMap.new