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, 2 insertions, 2 deletions

diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index 885dc0ecba..f07be831d4 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -105,7 +105,7 @@ module ActionDispatch # :nodoc:
 
       def body
         @str_body ||= begin
-          buf = "".dup
+          buf = +""
           each { |chunk| buf << chunk }
           buf
         end
diff --git a/actionpack/lib/action_dispatch/http/url.rb b/actionpack/lib/action_dispatch/http/url.rb
index 35ba44005a..db6d8188d3 100644
--- a/actionpack/lib/action_dispatch/http/url.rb
+++ b/actionpack/lib/action_dispatch/http/url.rb
@@ -157,7 +157,7 @@ module ActionDispatch
           subdomain  = options.fetch :subdomain, true
           domain     = options[:domain]
 
-          host = "".dup
+          host = +""
           if subdomain == true
             return _host if domain.nil?