Fix RFC 2069 mode digest authentication

[the-ress]

Summary

Fixes a typo that breaks old RFC 2069 mode digest auth.

The issue is that HA1 is added both on line 283, and again on line 293. (According to RFC, it should be there only once.)

Also mentioned in #2788 (comment)

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[tomchristie]

I appreciated your "this branch is RFC ..." comment in #2600 (comment) - perhaps we should include those?

Also the link to the curl code is super helpful, thanks.

[the-ress]

I made the discussed changes and added a few tests.

[tomchristie]

Looking wonderful, thanks. ✨😊✨

You'll also want to update the CHANGELOG.md with an entry pointing at this PR.

[the-ress]

You'll also want to update the CHANGELOG.md with an entry pointing at this PR.

Done.