Update deprecated Module#parent call

Author: tiagocasanovapt

lib/action_controller/serialization.rb

@@ -23,7 +23,7 @@ def serialization_scope(scope)
     end
 
     def namespace_for_serializer
-      @namespace_for_serializer ||= self.class.parent unless self.class.parent == Object
+      @namespace_for_serializer ||= self.class.module_parent unless self.class.module_parent == Object
     end
 
     def serialization_scope