Withers don't move on their own

Steps to Reproduce:

1. Summon a Wither;
2. Watch its movement pattern.

Observed Results:
The Wither stays in the same spot, throwing skulls at other mobs and only moves on its own when at half health.

Expected Results:
The Wither is able to move on its own without the need of the dash attack.

Notes: Unsure whether this behavior was present in previous versions or not.