Wither doesn't generate health when in minecart

What I expected to happen was the Wither keep generating health even in minecart

But the Wither doesn't generate health in minecart and become really easy to defeat, because the Wither only have half of full health after the invincibility mode done

To trigger this bug :
1. Create a small rail
2. Get a minecart and a minecart with furnace (or a minecart but with an active powered rail)
3. Create a Wither on top of the track while the minecart is moving (Note that the 2 other head is because other 13w04a bug)
4. The Wither get inside the minecart and doesn't generate health
5. The Wither explode, destroying the rail and the minecart, but have less health than normally

*Work in survival too