fixed issue with rendering Hash which appears in rails 4.2.0.beta4

lib/active_model/serializer.rb

@@ -56,7 +56,7 @@ end
       attr_reader :key_format
 
       def serializer_for(resource, options = {})
-        if resource.respond_to?(:each)
+        if resource.respond_to?(:each) && !resource.is_a?(Hash)
           if Object.constants.include?(:ArraySerializer)
             ::ArraySerializer
           else

test/integration/action_controller/namespaced_serialization_test.rb

@@ -19,6 +19,10 @@ module ActionController
         def render_comments
           render json: [Comment.new(content: 'Comment 1')]
         end
+
+        def render_hash
+          render json: {message: 'not found'}, status: 404
+        end
       end
 
       tests TestNamespace::MyController
@@ -42,6 +46,11 @@ module ActionController
         get :render_comments
         assert_serializer CommentSerializer
       end
+
+      def test_render_hash_regression
+        get :render_hash
+        assert_equal JSON.parse(response.body), {'message' => 'not found'}
+      end
     end
 
     class OptionNamespacedSerializationTest < ActionController::TestCase
@@ -93,4 +102,4 @@ module ActionController
     end
 
   end
-end
+end