WORLDGUARD-2857 - Reported by CubieX

When a player is sitting on a horse within a region with flag "mob-spawning: deny" set,
the horse will not spawn if he leaves the server and joins again later.
The horse will be gone.
WG will block the spawn because of the flag.

A possible solution could be to ignore the flag,
if the horse/donkey/mule that is about to be spawned is "tamed".