Add auto-dismounting from donkeys

[Grumpy_Nic]

Donkeys are not known to be trained battle horses. If somebody mounts a donkey there should be a chance (like 10%?) to automatically dismount again - showing the stubborness of the donkey and shaking off the rider.

[Daunt_Flockula]

  Would be really funny indeed but I would like to see an animated expression of the stubbornness, which is too much to ask for. Still, I like the idea.

[Molly]

I support this notion.