Trial chambers mobs (strays especially) Spawn on glass

Strays and other mobs can spawn on top of glass via a trial spawner.

Steps to Reproduce:

Method 1:

1. Build a large flat platform of glass
2. Place a trial spawner 1 block above it
3. Use a mob egg on the trial spawner
4. Enter survival to activate the spawner

Method 2:

1. Load into this world:
   spawner test.zip
2. Enter survival
3. Wait for mobs to spawn on the glass