Use String#+@ before mutating the result of Symbol#to_s

* String#+@ is available since Ruby 2.3.
* See the upstream Ruby change making Symbol#to_s return a frozen String
  for efficiency: https://github.com/ruby/ruby/pull/2437

activesupport/lib/active_support/ordered_options.rb

@@ -39,7 +39,7 @@ module ActiveSupport
     end
 
     def method_missing(name, *args)
-      name_string = name.to_s
+      name_string = +name.to_s
       if name_string.chomp!("=")
         self[name_string] = args.first
       else