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

```

2 files changed, 6 insertions, 6 deletions

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")