Fix deprecation warning

Fix the "Calling deprecated ArraySerializer... Please use
CollectionSerializer" warning that appeared when running the specs. This
happened because on of the test called ArraySerializer instead of
CollectionSerializer.

test/adapter/json_api/api_objects/relationship_test.rb

@@ -37,7 +37,7 @@ module ActiveModel
 
             def test_relationship_with_data_array
               posts = [Post.new(id: 1), Post.new(id: 2)]
-              @serializer = ActiveModel::Serializer::ArraySerializer.new(posts)
+              @serializer = ActiveModel::Serializer::CollectionSerializer.new(posts)
               @author.posts = posts
               @author.blog = nil
               expected = {