Add _format_download_uri_for_extension method #108
In #105 we saw that there should be a method to leverage the URI building functionality of the Gutenberg library for non-text resources. This pull request implements
This is not an official API (whence the underscore prefix) so no strict requirement to always maintain this function, but it's useful functionality to expose nevertheless given that we already have it in the code base.
When working on this change, I also noticed that we weren't correctly handling the case where the mirror root URL was changed in a single session so I also added a fix for that.
When verifying the refactor, I saw some odd behavior when running the tests locally, so this pull request also improves the developer experience for the tests:
Would it be better to instead call