TNT doesnt blow up grass block.

Put the summary of the bug you're having here

What I expected to happen was...:
That the TNT would blow up a big crater.

What actually happened was...:
it blows a big hole in the ground while leaving the grass blocks still covering the hole like there's no hole at all.

Steps to Reproduce:
1. Dig down in a straight shaft downwards.
2.Fill the shaft up with TNT
3. Ignite the TNT. You will notice that the grass has remained over the large hole you made.