Zombie Pigmen catch a little fire in daylight

Put the summary of the bug you're having here

What I expected to happen was...:
Nothing, just spawning...

What actually happened was...:
Catching a little fire and then off! (happened and in 1.2.5 or older

Steps to Reproduce:
1. Put a step by step guide on how to find this bug.
2. Start a world on Creative
3. Spawn a Zombie Pigman in DAY
4. See it catching fire but not getting hurt.

Attachements : I finnally took some but not good but are enough!