Minecarts do not move with forward/backwards key

Minecarts no longer gain momentum from the user pressing the forward or backwards keys.

Steps to replicate:

Open minecraft version 15w43c.
Create new a creative world with default settings.
Place at least 10 rails with a minecart on it.
Enter mineart and press 'w' or 's' (forward/backwards keys)
Observe minecarts do not gain momentum from key presses.

This can be replicated every time.