ruby 2.0 makes protected methods return false for respond_to, so pass true as the second param

activemodel/lib/active_model/attribute_methods.rb

@@ -223,7 +223,7 @@ def define_attribute_method(attr_name)
           unless instance_method_already_implemented?(method_name)
             generate_method = "define_method_#{matcher.method_missing_target}"
 
-            if respond_to?(generate_method)
+            if respond_to?(generate_method, true)
               send(generate_method, attr_name)
             else
               define_optimized_call generated_attribute_methods, method_name, matcher.method_missing_target, attr_name.to_s