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/actionview/test/activerecord/form_helper_activerecord_test.rb b/actionview/test/activerecord/form_helper_activerecord_test.rb
index 1472ee8def..6c44cb9e6c 100644
--- a/actionview/test/activerecord/form_helper_activerecord_test.rb
+++ b/actionview/test/activerecord/form_helper_activerecord_test.rb
@@ -57,7 +57,7 @@ class FormHelperActiveRecordTest < ActionView::TestCase
   private
 
     def hidden_fields(method = nil)
-      txt = %{<input name="utf8" type="hidden" value="&#x2713;" />}.dup
+      txt = +%{<input name="utf8" type="hidden" value="&#x2713;" />}
 
       if method && !%w(get post).include?(method.to_s)
         txt << %{<input name="_method" type="hidden" value="#{method}" />}
@@ -67,7 +67,7 @@ class FormHelperActiveRecordTest < ActionView::TestCase
     end
 
     def form_text(action = "/", id = nil, html_class = nil, remote = nil, multipart = nil, method = nil)
-      txt =  %{<form accept-charset="UTF-8" action="#{action}"}.dup
+      txt =  +%{<form accept-charset="UTF-8" action="#{action}"}
       txt << %{ enctype="multipart/form-data"} if multipart
       txt << %{ data-remote="true"} if remote
       txt << %{ class="#{html_class}"} if html_class