[1.4][Singleplayer] Farming turtles will break grass blocks directly below or above them instead of tilling

How to reproduce the bug:

1. Place a grass block
2. Place a farming turtle (the hoe turtle) on top
3. Give the turtle the "turtle.digDown()" command
4. The grass block will be removed from the world and a dirt block will appear in the turtle's inventory

The turtle tills grass (as intended) if the grass block is in front of the turtle.

Tested on the Feed The Beast Direwolf20 v4 pack. This means ComputerCraft 1.48 and MiscPeripherals 2.2.

to till down, you want to be one block higher, with an empty space between you and the grass/dirt. I believe this is intended behavior. Think of it like the block you'd plant in is the square you're tilling, not the actual dirt block you plant on.