SASLHandler fails on binary data for mechanism GSSAPI
Reported by thpo:
When using gajim with Kerberos authorization, it fails to utf8-decode and print the data received, because it is binary.
The SASLHandler in nbxmpp/auth_nb.py assumes the data transmitted by the SASL mechanism is somehow printable.
Do not utf8-decode or print the data transmitted for GSSAPI.