Pigs struck by lightning have swords.

I was playing around with summoning lightning and noticed that when a pig got struck by lightning and changed into a zombie pigman, it had a sword. I know that didn't used to happen in 1.6, I'm not sure about 1.7 and don't know how to check. Trying it on other pigs, I had the same result. I don't know whether this is a bug or whether it was meant to happen, but it wasn't on the update notes and it doesn't really make sense - where does the sword come from?
What I expected to happen was the zombie pigman would have no sword.
What actually happened was it had a gold sword, though where it got the gold from I can't imagine.
Steps to Reproduce:
1. Spawn a pig
2. Do /summon LighningBolt
3. Observe the sword appearing from nowhere.