Sheep eating grass does not cause a block update. Tested in 13w16a and 1.5.1

What I expected to happen was...:
The grass block turning to dirt when a sheep ate it to activate a BUD.

What actually happened was...:
The BUD did not activate. The BUD does work, verified by waiting for the grass to grow back.

Steps to Reproduce:
Place a piston facing upwards, place grass block on top.
Place another block adjacent to the grass block, and power it (a BUD switch)
Block off the grass block so a sheep cannot jump off.
Spawn sheep on grass block and shear.
Wait for sheep to eat grass block.
BUD piston does not extend.