Add autoload for ActionController::TestRequest

This constant is referenced in
actionpack/lib/action_dispatch/testing/assertions/routing.rb,
which doesn't directly depend on ActionController::TestCase.

Author: eugeneius

actionpack/lib/action_controller.rb

@@ -66,8 +66,11 @@ module ActionController
     autoload :ApiRendering
   end
 
-  autoload :TestCase,           "action_controller/test_case"
-  autoload :TemplateAssertions, "action_controller/test_case"
+  autoload_at "action_controller/test_case" do
+    autoload :TestCase
+    autoload :TestRequest
+    autoload :TemplateAssertions
+  end
 end
 
 # Common Active Support usage in Action Controller