grass spawns on grass blockes and gravel when world created

Put the summary of the bug you're having here

What I expected to happen was...:grasses not spawning on grass blocks and gravel
Describe what you thought should happen here

What actually happened was...:grasses spawned on grass blocks and gravel
Describe what happened here

Steps to Reproduce:
1. create a new  unmodified world
2. ...
3. ...