Checking if we have to run this authentication method once it's already passed
to imaplib2 is too late. Make the checks at correct time, before we try the
authentication method. IOW, before we call
imapobj.authenticate('XOAUTH2', self.__xoauth2handler)
Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/442
Tested-by: Klemens Schölhorn <klemens@schoelhorn.eu>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>