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, 7 insertions, 7 deletions

diff --git a/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb b/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
index 077a83b112..5f5fdbc66a 100644
--- a/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
+++ b/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
@@ -23,7 +23,7 @@ module ActionDispatch
         if clean_params.empty?
           "None"
         else
-          PP.pp(clean_params, "".dup, 200)
+          PP.pp(clean_params, +"", 200)
         end
       end
 
diff --git a/actionpack/lib/action_dispatch/middleware/debug_locks.rb b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
index 03760438f7..d39377f174 100644
--- a/actionpack/lib/action_dispatch/middleware/debug_locks.rb
+++ b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
@@ -63,19 +63,19 @@ module ActionDispatch
 
         str = threads.map do |thread, info|
           if info[:exclusive]
-            lock_state = "Exclusive".dup
+            lock_state = +"Exclusive"
           elsif info[:sharing] > 0
-            lock_state = "Sharing".dup
+            lock_state = +"Sharing"
             lock_state << " x#{info[:sharing]}" if info[:sharing] > 1
           else
-            lock_state = "No lock".dup
+            lock_state = +"No lock"
           end
 
           if info[:waiting]
             lock_state << " (yielded share)"
           end
 
-          msg = "Thread #{info[:index]} [0x#{thread.__id__.to_s(16)} #{thread.status || 'dead'}]  #{lock_state}\n".dup
+          msg = +"Thread #{info[:index]} [0x#{thread.__id__.to_s(16)} #{thread.status || 'dead'}]  #{lock_state}\n"
 
           if info[:sleeper]
             msg << "  Waiting in #{info[:sleeper]}"
diff --git a/actionpack/lib/action_dispatch/middleware/ssl.rb b/actionpack/lib/action_dispatch/middleware/ssl.rb
index 190e54223e..9c9ccfa16f 100644
--- a/actionpack/lib/action_dispatch/middleware/ssl.rb
+++ b/actionpack/lib/action_dispatch/middleware/ssl.rb
@@ -102,7 +102,7 @@ module ActionDispatch
 
       # https://tools.ietf.org/html/rfc6797#section-6.1
       def build_hsts_header(hsts)
-        value = "max-age=#{hsts[:expires].to_i}".dup
+        value = +"max-age=#{hsts[:expires].to_i}"
         value << "; includeSubDomains" if hsts[:subdomains]
         value << "; preload" if hsts[:preload]
         value
@@ -141,7 +141,7 @@ module ActionDispatch
         host = @redirect[:host] || request.host
         port = @redirect[:port] || request.port
 
-        location = "https://#{host}".dup
+        location = +"https://#{host}"
         location << ":#{port}" if port != 80 && port != 443
         location << request.fullpath
         location