Withers can be pushed during their spawning animations

The Bug:

Withers can be pushed during their spawning animations.

Steps to Reproduce:

1. Summon a wither by placing soul sand and wither skeletons skulls.
2. Attempt to push the wither during its spawning animation.

Observed Behavior:

Withers can be pushed.

Expected Behavior:

Withers would not be able to be pushed.