Skeletons wielding swords do not fight other mobs

Skeletons holding swords will not fight a mob that hurts them other than a player. Skeletons with bows, however, will fight other mobs back. This applies to Wither Skeletons too, since they share the same entity ID.

Steps to Reproduce:
1. Make a new Creative world.
2. Spawn a few skeletons (make sure its night).
3. Give some of them swords and leave some with their bows.
4. Spawn an Iron Golem near the skeletons.
5. Observe how the skeletons with swords do not look at the Iron Golem when it attacks them but the bow-wielding skeletons do look at it.