This change makes it so that EntityHuman#setPassengerOf(Entity) invokes
its parent method when leaving vehicles so that VehicleExitEvent is fired
for players leaving vehicles.
This change also fixes BUKKIT-2110, making it so VehicleExitEvent
correctly handles cancellation. The implementation of VehicleExitEvent
completely ignored the cancellation state of the event, making it so that
cancelling the event had no effect. Cancelling a VehicleExitEvent now
causes the entity to remain inside of the vehicle, with no visual stutter.
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