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, 11 insertions, 11 deletions

diff --git a/activesupport/test/core_ext/object/instance_variables_test.rb b/activesupport/test/core_ext/object/instance_variables_test.rb
index a3d8daab5b..cf1fe5dfa4 100644
--- a/activesupport/test/core_ext/object/instance_variables_test.rb
+++ b/activesupport/test/core_ext/object/instance_variables_test.rb
@@ -19,7 +19,7 @@ class ObjectInstanceVariableTest < ActiveSupport::TestCase
   end
 
   def test_instance_exec_passes_arguments_to_block
-    assert_equal %w(hello goodbye), "hello".dup.instance_exec("goodbye") { |v| [self, v] }
+    assert_equal %w(hello goodbye), (+"hello").instance_exec("goodbye") { |v| [self, v] }
   end
 
   def test_instance_exec_with_frozen_obj
@@ -27,7 +27,7 @@ class ObjectInstanceVariableTest < ActiveSupport::TestCase
   end
 
   def test_instance_exec_nested
-    assert_equal %w(goodbye olleh bar), "hello".dup.instance_exec("goodbye") { |arg|
+    assert_equal %w(goodbye olleh bar), (+"hello").instance_exec("goodbye") { |arg|
       [arg] + instance_exec("bar") { |v| [reverse, v] } }
   end
 end
diff --git a/activesupport/test/core_ext/string_ext_test.rb b/activesupport/test/core_ext/string_ext_test.rb
index b8de16cc5e..81299e5b58 100644
--- a/activesupport/test/core_ext/string_ext_test.rb
+++ b/activesupport/test/core_ext/string_ext_test.rb
@@ -245,8 +245,8 @@ class StringInflectionsTest < ActiveSupport::TestCase
   end
 
   def test_string_squish
-    original = %{\u205f\u3000 A string surrounded by various unicode spaces,
-      with tabs(\t\t), newlines(\n\n), unicode nextlines(\u0085\u0085) and many spaces(  ). \u00a0\u2007}.dup
+    original = +%{\u205f\u3000 A string surrounded by various unicode spaces,
+      with tabs(\t\t), newlines(\n\n), unicode nextlines(\u0085\u0085) and many spaces(  ). \u00a0\u2007}
 
     expected = "A string surrounded by various unicode spaces, " \
       "with tabs( ), newlines( ), unicode nextlines( ) and many spaces( )."
@@ -378,8 +378,8 @@ class StringInflectionsTest < ActiveSupport::TestCase
   end
 
   def test_truncate_multibyte
-    assert_equal "\354\225\204\353\246\254\353\236\221 \354\225\204\353\246\254 ...".dup.force_encoding(Encoding::UTF_8),
-      "\354\225\204\353\246\254\353\236\221 \354\225\204\353\246\254 \354\225\204\353\235\274\353\246\254\354\230\244".dup.force_encoding(Encoding::UTF_8).truncate(10)
+    assert_equal (+"\354\225\204\353\246\254\353\236\221 \354\225\204\353\246\254 ...").force_encoding(Encoding::UTF_8),
+      (+"\354\225\204\353\246\254\353\236\221 \354\225\204\353\246\254 \354\225\204\353\235\274\353\246\254\354\230\244").force_encoding(Encoding::UTF_8).truncate(10)
   end
 
   def test_truncate_should_not_be_html_safe
@@ -400,7 +400,7 @@ class StringInflectionsTest < ActiveSupport::TestCase
   end
 
   def test_remove!
-    original = "This is a very good day to die".dup
+    original = +"This is a very good day to die"
     assert_equal "This is a good day to die", original.remove!(" very")
     assert_equal "This is a good day to die", original
     assert_equal "This is a good day", original.remove!(" to ", /die/)
@@ -733,7 +733,7 @@ end
 
 class OutputSafetyTest < ActiveSupport::TestCase
   def setup
-    @string = "hello".dup
+    @string = +"hello"
     @object = Class.new(Object) do
       def to_s
         "other"
@@ -809,7 +809,7 @@ class OutputSafetyTest < ActiveSupport::TestCase
   end
 
   test "Concatting safe onto unsafe yields unsafe" do
-    @other_string = "other".dup
+    @other_string = +"other"
 
     string = @string.html_safe
     @other_string.concat(string)
@@ -832,7 +832,7 @@ class OutputSafetyTest < ActiveSupport::TestCase
   end
 
   test "Concatting safe onto unsafe with << yields unsafe" do
-    @other_string = "other".dup
+    @other_string = +"other"
     string = @string.html_safe
 
     @other_string << string
@@ -888,7 +888,7 @@ class OutputSafetyTest < ActiveSupport::TestCase
   test "Concatting an integer to safe always yields safe" do
     string = @string.html_safe
     string = string.concat(13)
-    assert_equal "hello".dup.concat(13), string
+    assert_equal (+"hello").concat(13), string
     assert_predicate string, :html_safe?
   end