Tested on sailfin v2.0 ((v2.1 Patch06)(9.1_02 Patch12)) (build b31-fcs) against a 3rd party client Nimbuzz.
If a client incorrectly responds to a 401 authentication challenge from the sailfin server with an ACK with
the same branch ID the sailfin server treats this as a re-transmission request and responds with the 401
The ACK should only be matched to the original transaction if it's an INVITE method. So in this case it
should be ignored.