(Bug 4829) Fix authenticate() call and returned status

Two things:
* Fixed the method call (oh perl, you let us shoot ourselves in the foot
  sometimes)
* Explicitly set the returned status code instead of just the status
* line (see Bug 4832 for more info)
Merge pull request #209 from afuna/auth-digest-bug4829

(Bug 4829) Fix authenticate() call and returned status