Add custom_params to completion_route

Since the method `completion_route` is used instead of the path method it doesn't support setting parameters. It would be nice if we could pass parameters while still using the `completion_route` method.

This is a remake of #5658 to be merged into `master`
Unused method argument - `response`. If it's necessary, use `_` or `_response` as an argument name to indicate that it won't be used. You can also write as `cancel(*)` if you want the method to accept any arguments but don't care about them.