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`
Redundant use of `Object#to_s` in interpolation.<br>Line is too long. [109/80]<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.