Grass can spawn on magma and netherack at ruined portals.

What I expected to happen was...:
Grass should not be able to spawn on magma or netherack.

What actually happened was...:
Grass spawned on magma and netherack when creating new worlds.

Steps to Reproduce:
1. Create new world
2. Find ruined portal in overworld
3. Grass has a chance to spawn on magma or netherack