Snow layers cannot be placed on top of mud

The bug

Snow layers are incapable of being placed on top of mud. This is inconsistent with soul sand, a block of identical hitbox size and shape, which accepts snow layers just fine.

How to reproduce

1. Obtain snow layers
2. Try placing a snow layer on soul sand
3. Try placing a snow layer on mud

Expected results

As soul sand and mud are full cubes, snow could be placed on both of them.

Actual results

Snow layers can only be placed on soul sand, and are completely rejected by mud.