On older worlds, mobs don't spawn.

On older worlds, mobs don't spawn.

I made a world in the furnace update,
and mobs spawned like crazy, but after
the spider/skeleton update they were all
gone.

Steps to Reproduce:
1. Find a device with an old update (preferably the furnace update)
2. Create a world
3. Update it to the current version
4. No mobs