How can I test an ActionDispatch-based gem?

I recently published my first gem, ensure_subdomain, which adds a few helper methods to ActionDispatch for stupid-simple subdomain redirection.

I have tests for the main class, which handles the request matching, and whathaveyou, but I don’t have any coverage for the one method that interacts with existing AD logic.

How do I go about testing this one bit? I’m using Minitest; do I need to use Rspec?