Generated snow does not update grass beneath it

When generating terrain with snow (like mountains), the grass block under the snow is not updated to have the `snowy=true` property

As you can see in the image, the grass block on the left doesn't have the correct state. The one on the right is hand placed for comparison

Steps to reproduce:
1. Create a new world with default preset
2. Find a snowy biome
3. Look at the grass beneath the snow layer