remove the mounted_helpers respond_to check

It always responds to mounted_helpers now

actionpack/lib/action_dispatch/testing/integration.rb

@@ -189,7 +189,7 @@ def initialize(app)
         if app.respond_to?(:routes)
           singleton_class.class_eval do
             include app.routes.url_helpers
-            include app.routes.mounted_helpers if app.routes.respond_to?(:mounted_helpers)
+            include app.routes.mounted_helpers
           end
         end