[rails/] Add frozen_string_literal to json and plaintext controllers (#8486)

Frozen strings are a bit faster.

frameworks/Ruby/rails/app/controllers/application_controller.rb

@@ -1,4 +1,5 @@
 # frozen_string_literal: true
+
 class ApplicationController < ActionController::Base
   before_action :add_header
 

frameworks/Ruby/rails/app/controllers/application_controller_metal.rb

@@ -1,4 +1,5 @@
 # frozen_string_literal: true
+
 class ApplicationControllerMetal < ActionController::Metal
   def add_headers
     response.set_header('Server', 'rails')

frameworks/Ruby/rails/app/controllers/json_controller.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class JsonController < ApplicationControllerMetal
   def index
     add_headers

frameworks/Ruby/rails/app/controllers/plaintext_controller.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class PlaintextController < ApplicationControllerMetal
   def index
     add_headers