Wither won't move in the new 1.9 snapshot

Put the summary of the bug you're having here

What I expected to happen was the wither would move around in attack animals
Describe what you thought should happen here

What actually happened was when it spawned in, it did not move at all:
Describe what happened here it spawned in, but did not move.

Steps to Reproduce:
1. Get the wither heads and soul sand
2. spawn the wither
3. Watch it not move