Prevent unbuffered responses from being deferred.

401 responses matching the shouldIgnoreURL criteria will not be buffered
and as such they should be be returned as a deferred promise but rather
they should be rejected.

By rejecting instead of deferring we allow the client to properly detect
failed login attempts in the login form. e.g. 401 from /api/auth
properly returns a 401 immediately upon improper credentials.