Include Serializer._type in collection serializer json_key cascade

test/collection_serializer_test.rb

@@ -3,6 +3,10 @@ require 'test_helper'
 module ActiveModel
   class Serializer
     class CollectionSerializerTest < ActiveSupport::TestCase
+      MessagesSerializer = Class.new(ActiveModel::Serializer) do
+        type 'messages'
+      end
+
       def setup
         @comment = Comment.new
         @post = Post.new

test/fixtures/poro.rb

@@ -183,12 +183,6 @@ PaginatedSerializer = Class.new(ActiveModel::Serializer::CollectionSerializer) d
   end
 end
 
-MessagesSerializer = Class.new(ActiveModel::Serializer) do
-  def json_key
-    'messages'
-  end
-end
-
 AlternateBlogSerializer = Class.new(ActiveModel::Serializer) do
   attribute :id
   attribute :name, key: :title