Fix for TypeError: can't compare datetime to bool

I repeatedly got `TypeError: can't compare datetime.datetime to bool` for this line. The consumer needed to be restarted for each exception, making Huey unusable. If a number of such tasks were queued, the same number of consumer restarts were required.

Not entirely sure whether `revoke_until` or `dt` was the bool in this case, but this should logically be a safe fix.