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, 2 insertions, 2 deletions

diff --git a/activestorage/test/models/filename_test.rb b/activestorage/test/models/filename_test.rb
index 88405e41c0..715116309f 100644
--- a/activestorage/test/models/filename_test.rb
+++ b/activestorage/test/models/filename_test.rb
@@ -30,8 +30,8 @@ class ActiveStorage::FilenameTest < ActiveSupport::TestCase
   end
 
   test "sanitize transcodes to valid UTF-8" do
-    { "\xF6".dup.force_encoding(Encoding::ISO8859_1) => "ö",
-      "\xC3".dup.force_encoding(Encoding::ISO8859_1) => "Ã",
+    { (+"\xF6").force_encoding(Encoding::ISO8859_1) => "ö",
+      (+"\xC3").force_encoding(Encoding::ISO8859_1) => "Ã",
       "\xAD" => "�",
       "\xCF" => "�",
       "\x00" => "",