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/actionpack/lib/action_dispatch/journey/router/utils.rb b/actionpack/lib/action_dispatch/journey/router/utils.rb
index df3f79a407..3bbb187f5c 100644
--- a/actionpack/lib/action_dispatch/journey/router/utils.rb
+++ b/actionpack/lib/action_dispatch/journey/router/utils.rb
@@ -17,11 +17,11 @@ module ActionDispatch
         def self.normalize_path(path)
           path ||= ""
           encoding = path.encoding
-          path = "/#{path}".dup
+          path = +"/#{path}"
           path.squeeze!("/".freeze)
           path.sub!(%r{/+\Z}, "".freeze)
           path.gsub!(/(%[a-f0-9]{2})/) { $1.upcase }
-          path = "/".dup if path == "".freeze
+          path = +"/" if path == "".freeze
           path.force_encoding(encoding)
           path
         end
@@ -32,7 +32,7 @@ module ActionDispatch
           ENCODE   = "%%%02X".freeze
           US_ASCII = Encoding::US_ASCII
           UTF_8    = Encoding::UTF_8
-          EMPTY    = "".dup.force_encoding(US_ASCII).freeze
+          EMPTY    = (+"").force_encoding(US_ASCII).freeze
           DEC2HEX  = (0..255).to_a.map { |i| ENCODE % i }.map { |s| s.force_encoding(US_ASCII) }
 
           ALPHA = "a-zA-Z".freeze