prevent JOSE algorithm type confusion

[bmw]

Moved here from certbot/certbot#332.

This isn't an issue for Certbot but could be for other projects using josepy.

[github-actions]

We've made a lot of changes to Josepy since this issue was opened. If you still have this issue with an up-to-date version of Josepy, can you please add a comment letting us know? This helps us to better see what issues are still affecting our users. If there is no activity in the next 30 days, this issue will be automatically closed.