boat collision boxes repel players like mobs

In 1.8, you could walk on boats. In the snapshots, attempting to stand on a boat, repels the player. This is because the collision boxes of boats are treated like mobs, while in the past they were treated separately.

To Reproduce:

1. Create a super flat world of solid blocks
2. Place a boat
3. Attempt to stand on the boat (you can see the effect best in F3+B mode)