Mobs can spawn on scaffolding

The bug

Mobs can spawn on scaffolding. As the top face of scaffolding is not necessarily solid (you can go through it), it is questionable whether this is intended.

Also, this is inconvenient if you need to take care of your scaffolding being spawn-proof when you want to focus on building your build.

To reproduce

1. Create a new superflat world with the preset

   minecraft:bedrock,minecraft:scaffolding;minecraft:plains;
   

2. Run /time set midnight
3. Mobs will spawn