Fix bug in renegotiating a nonce with the server

If a session runs long enough (without constant activity) then the server can
expire the nonce the session has negotiated. If that happens the session will
get a new 401 response which we were immediately returning to the user. A user
would then have to essentially reinitialize session.auth each time they get an
unexpected 401.

Also, there's no need for setattr calls when we can simply assign the
attribute on the instance.
Merge pull request #2389 from sigmavirus24/bug/fix-digest-auth

Fix bug in renegotiating a nonce with the server