Use class_attribute to allow changing statuses per-responder

If an app happens to have multiple responders on different controllers
(which is entirely possible), they may not want to share the same set of
statuses for responses. `cattr_accessor` would make that a global value
across all of the apps responders, while `class_attribute` can be
changed on each class individually if necessary (while still respecting
the inherited values if any is set), making it much more flexible.

Also disable instance writer and predicate methods, we only need access
to read those values from the instance.

lib/action_controller/responder.rb

@@ -120,8 +120,9 @@ module ActionController # :nodoc:
  #
  # Using <code>respond_with</code> with a block follows the same syntax as <code>respond_to</code>.
  class Responder
- cattr_accessor :error_status, default: :ok
- cattr_accessor :redirect_status, default: :found
+ class_attribute :error_status, default: :ok, instance_writer: false, instance_predicate: false
+ class_attribute :redirect_status, default: :found, instance_writer: false, instance_predicate: false
+
  attr_reader :controller, :request, :format, :resource, :resources, :options
 
  DEFAULT_ACTIONS_FOR_VERBS = {