Merge pull request #1048 from bf4/cleanup_meta_in_adapter

Let FlattenJson adapter decide it doesn't include meta

lib/active_model/serializer/adapter/flatten_json.rb

@@ -6,6 +6,13 @@ module ActiveModel
           super
           @result
         end
+
+        private
+
+        # no-op: FlattenJson adapter does not include meta data, because it does not support root.
+        def include_meta(json)
+          json
+        end
       end
     end
   end