Use a different controller in different tests

A number of test were defining and using the same controller
MyController = Class.new(ActionController::Base)
which was causing some state to leak across tests.

test/action_controller/serialization_test.rb

@@ -3,7 +3,7 @@ require 'test_helper'
 module ActionController
   module Serialization
     class ImplicitSerializerTest < ActionController::TestCase
-      class MyController < ActionController::Base
+      class ImplicitSerializationTestController < ActionController::Base
         def render_using_implicit_serializer
           @profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
           render json: @profile
@@ -152,7 +152,7 @@ module ActionController
         end
       end
 
-      tests MyController
+      tests ImplicitSerializationTestController
 
       # We just have Null for now, this will change
       def test_render_using_implicit_serializer