Use active record error attribute_names intead errors.keys if available

Author: cristianstu

lib/jsonapi/rails/serializable_active_model_errors.rb

@@ -25,14 +25,20 @@ def initialize(exposures)
       end
 
       def as_jsonapi
-        @errors.keys.flat_map do |key|
+        error_keys.flat_map do |key|
           @errors.full_messages_for(key).map do |message|
             SerializableActiveModelError.new(field: key, message: message,
                                              pointer: @reverse_mapping[key])
               .as_jsonapi
           end
         end
       end
+
+      private
+
+      def error_keys
+        @errors.respond_to?(:attribute_names) ? @errors.attribute_names : @errors.keys
+      end
     end
   end
 end