Fix loading ActionMailbox::BaseController when CSRF protection is disabled

When `default_protect_from_forgery` is false, `verify_authenticity_token`
callback does not define and `skip_forgery_protection` raise exception.

Fixes rails#34837.

Author: y-yagi

actionmailbox/app/controllers/action_mailbox/base_controller.rb

@@ -3,7 +3,7 @@
 module ActionMailbox
   # The base class for all Action Mailbox ingress controllers.
   class BaseController < ActionController::Base
-    skip_forgery_protection
+    skip_forgery_protection if default_protect_from_forgery
 
     before_action :ensure_configured