Endermen don't open their mouth when attacked by other mobs

Endermen will not open their mouth or shake when attacked by an Iron Golem or a Wither.

What I expected to happen was...:
The Enderman to open his mouth and shake in anger when an Iron Golem attacked it.

What actually happened was...:
The Enderman kept its mouth shut and was not shaking.

Steps to Reproduce:
1. Create an Iron Golem in creative
2. Spawn an Enderman close to the Iron Golem (Make sure it's night).
3. When the Iron Golem attacks the Enderman, he won't open his mouth or shake.