Resolution: Works As Intended
I spawned a Witherboss next to a moving minecart (actually I spawned the Wither in water but I could have used gamerule mobGriefing false) So after I spawned the Witherboss I expected it to go into the minecart, like it used to do in snapshot 13w04a and before.
Instead the Witherboss did not go into the minecart, not even when I pushed the minecart into the wither / pushed the wither into the minecart.
Steps to reproduce:
1. Spawn a Wither. (use gamerule mobGriefing false for the explosion)
2. Build a minecarttrack close to the Wither.
3. Notice that the Wither does not go into the minecart.