Avoid misuse of underscore argument

Per convention, underscore-only argument names should be used for unused parameters.

actionpack/lib/action_controller/metal/strong_parameters.rb

@@ -158,8 +158,8 @@ def permitted?
     def permit!
       each_pair do |key, value|
         value = convert_hashes_to_parameters(key, value)
-        Array.wrap(value).each do |_|
-          _.permit! if _.respond_to? :permit!
+        Array.wrap(value).each do |v|
+          v.permit! if v.respond_to? :permit!
         end
       end