Fixing the :hide_documentation_path option for prefixed APIs

I discovered an issue that when mounting an API like `/api/v1/<some_endpoint>` the :hide_documentation_path just didn't do anything. The reason was that the piece of code with `routes.reject!` only checked the top most path, and didn't recurse down.

I tried myself at a fix and also added two more tests. I don't think it is optimal yet but it works for me.