Slime block doesn't break blocks (e.g melon) when pulled.

Put the summary of the bug you're having here

What I expected to happen was...:
When a slime block is pulled by a piston while in contact with a melon, pumpkin or dragon egg, I expected the block to break and drop its item form; As this is the case if they are pulled by a standard sticky piston.

What actually happened was...:
The block was ignored in the same way that bedrock or obsidian would have been.

Steps to Reproduce:
1. power a sticky piston with a slime block attached so it pushes the slime block.
2. place a melon (or a pumpkin/dragon egg) so it is in contact with the slime block
3. un-power piston