Allow authentication with headers in the Debian distribution APIs

What does this MR do and why?

The current doc uses --header "PRIVATE-TOKEN: <your_access_token>" to create a Debian distribution, but this is broken since initial MR.

This fixes this, and refactor the specs to improve coverage while reducing the number of tests 🤔!

From:

Finished in 3 minutes 41.2 seconds (files took 39.57 seconds to load)
736 examples, 0 failures

To:

Finished in 2 minutes 41.8 seconds (files took 36.49 seconds to load)
454 examples, 0 failures