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

```

3 files changed, 4 insertions, 4 deletions

diff --git a/actionpack/test/dispatch/debug_exceptions_test.rb b/actionpack/test/dispatch/debug_exceptions_test.rb
index 44b79c0e5d..37399cfd07 100644
--- a/actionpack/test/dispatch/debug_exceptions_test.rb
+++ b/actionpack/test/dispatch/debug_exceptions_test.rb
@@ -368,7 +368,7 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
     })
     assert_response 500
 
-    assert_includes(body, CGI.escapeHTML(PP.pp(params, "".dup, 200)))
+    assert_includes(body, CGI.escapeHTML(PP.pp(params, +"", 200)))
   end
 
   test "sets the HTTP charset parameter" do
diff --git a/actionpack/test/dispatch/prefix_generation_test.rb b/actionpack/test/dispatch/prefix_generation_test.rb
index 85ea04356a..7a7a201b11 100644
--- a/actionpack/test/dispatch/prefix_generation_test.rb
+++ b/actionpack/test/dispatch/prefix_generation_test.rb
@@ -13,7 +13,7 @@ module TestGenerationPrefix
     end
 
     def self.model_name
-      klass = "Post".dup
+      klass = +"Post"
       def klass.name; self end
 
       ActiveModel::Name.new(klass)
diff --git a/actionpack/test/dispatch/static_test.rb b/actionpack/test/dispatch/static_test.rb
index 6b69cd9999..d44aa00122 100644
--- a/actionpack/test/dispatch/static_test.rb
+++ b/actionpack/test/dispatch/static_test.rb
@@ -31,7 +31,7 @@ module StaticTests
   end
 
   def test_handles_urls_with_ascii_8bit
-    assert_equal "Hello, World!", get("/doorkeeper%E3E4".dup.force_encoding("ASCII-8BIT")).body
+    assert_equal "Hello, World!", get((+"/doorkeeper%E3E4").force_encoding("ASCII-8BIT")).body
   end
 
   def test_handles_urls_with_ascii_8bit_on_win_31j
@@ -39,7 +39,7 @@ module StaticTests
       Encoding.default_internal = "Windows-31J"
       Encoding.default_external = "Windows-31J"
     end
-    assert_equal "Hello, World!", get("/doorkeeper%E3E4".dup.force_encoding("ASCII-8BIT")).body
+    assert_equal "Hello, World!", get((+"/doorkeeper%E3E4").force_encoding("ASCII-8BIT")).body
   end
 
   def test_handles_urls_with_null_byte