Llamas can be equipped with a saddle using commands

Discovered while testing MC-274427.

The bug:

By using the /data and /item commands, llamas can have saddles equipped onto them. If a llama with a saddle is tamed, it can be controlled by the player riding it, just like a horse.

How to reproduce:

1. Execute the following command:

   /summon minecraft:llama ~ ~ ~ {Tame:1b,SaddleItem:{id:"minecraft:saddle",count:1}}

2. Ride the llama you just summoned.

3. Try to move around.

Expected result:

The llama wouldn't have a SaddleItem tag, and therefore would be unable to wear a saddle.

Observed result:

The llama can wear a saddle and be controlled by a player riding it.