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`
Use `||` instead of `or`.<br>Indent the right brace the same as the first position after the preceding left parenthesis.