Withers hitbox/collision box does not incorporate the two smaller heads

Withers cannot be hit where the other two heads are, including the "arms" connecting to the heads. The same applies to their collision box, as a Wither can fly through a one block hole upwards.

Steps to Reproduce:
1. Make a new Creative world.
2. Spawn a Wither.
3. When you try to hit the two smaller heads, notice how the Wither takes no damage.
4. Also, press F3+B to show entity collision boxes and notice the box on the Wither only includes the main head and spine.