DIGEST-MD5 response negotiation mishandles qop and quoted directives

[OskarEichler]

Problem

DIGEST-MD5 response construction can choose the wrong qop default/advertised option, escape quoted directive values incorrectly and lose empty directives. Servers with multiple qop values, quotes/backslashes or empty realms can reject otherwise valid authentication.

Prepared atomic commit a23cc925036aa6abf2a9ee5dbcfa1c995be91647 corrects qop selection/defaulting, directive escaping and empty-value preservation. Dual-Ruby RFC-style vectors and the full suite pass. No repository tests were modified.

[nevans]

@OskarEichler Please proofread your bot's issues and PRs before submitting them. If you wouldn't submit it as a human, you shouldn't let your bots do it on your behalf.

What I'm specifically referring to here is this: "Prepared atomic commit ....". If you've prepared an atomic commit, where is that commit?! This should've been a PR, not an issue (or at least an issue with a linked PR)!

Or "No repository tests were modified." Of course no tests were modified... because this is an issue, not a PR!