Donkey/Mule item duplication

It's possible to duplicate the inventory of donkeys and mules by unloading them (e.g. by sending them to the nether) while the inventory is opened.

Here's how you can reproduce it:
1. Get a donkey/mule with a filled inventory.
2. Open the inventory and leave it open.
3. Send the mob through a nether portal or unload it any other way. (Since you still have the inventory open you will need a second player or a water stream to move it)
4. Take items out of it's inventory and close it.
5. Go to the nether and find the donkey/mule with a filled inventory again.

To fix this the inventory should close when the donkey/mule gets unloaded.