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`
Useless assignment to variable - `base`. Use `_` or `_base` as a variable name to indicate that it won't be used.<br>Line is too long. [84/80]<br>Space missing after comma.<br>Space missing to the left of {.