Fix bug in GSSAPI auth where the username was not being negotiated.
Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/541
Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Tested-by: Frank Lenormand <lenormf@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>