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

```

1 files changed, 3 insertions, 3 deletions

diff --git a/activesupport/test/cache/behaviors/encoded_key_cache_behavior.rb b/activesupport/test/cache/behaviors/encoded_key_cache_behavior.rb
index da16142496..842400f4a3 100644
--- a/activesupport/test/cache/behaviors/encoded_key_cache_behavior.rb
+++ b/activesupport/test/cache/behaviors/encoded_key_cache_behavior.rb
@@ -6,7 +6,7 @@
 module EncodedKeyCacheBehavior
   Encoding.list.each do |encoding|
     define_method "test_#{encoding.name.underscore}_encoded_values" do
-      key = "foo".dup.force_encoding(encoding)
+      key = (+"foo").force_encoding(encoding)
       assert @cache.write(key, "1", raw: true)
       assert_equal "1", @cache.read(key)
       assert_equal "1", @cache.fetch(key)
@@ -18,7 +18,7 @@ module EncodedKeyCacheBehavior
   end
 
   def test_common_utf8_values
-    key = "\xC3\xBCmlaut".dup.force_encoding(Encoding::UTF_8)
+    key = (+"\xC3\xBCmlaut").force_encoding(Encoding::UTF_8)
     assert @cache.write(key, "1", raw: true)
     assert_equal "1", @cache.read(key)
     assert_equal "1", @cache.fetch(key)
@@ -29,7 +29,7 @@ module EncodedKeyCacheBehavior
   end
 
   def test_retains_encoding
-    key = "\xC3\xBCmlaut".dup.force_encoding(Encoding::UTF_8)
+    key = (+"\xC3\xBCmlaut").force_encoding(Encoding::UTF_8)
     assert @cache.write(key, "1", raw: true)
     assert_equal Encoding::UTF_8, key.encoding
   end