Double lock in digest

[Helyk]

Hello. I have ran into a problem where in digest.go#L262 da.Purge is called while da.mutex is locked, with da.Purge itself trying to acquire lock on it.

This causes double lock and all further digest authentication requests hang until they time out.

This can be fixed either by removing lines 74, 75, 92 and 99, so that da.Purge is always called when da.mutex is locked but is not trying to acquire lock itself, or by adding da.mutex.Unlock() and da.mutex.Lock() around line 262 so that da.Purge is only called when da.mutex is not locked.