Summary:
While updating from 1.7.4 -> 14w02c -> 14w03b is normal, witch huts spawn normal mobs after updating to 14w04b (and 14w05b).

How to reproduce:
1. Create a world (default world type) in version 1.7.4 with the seed -3784590470294277123
2. At (-448,-1472) there is a witch hut, which spawns witches normally
3. Open the world in version 14w04b (or 14w05b)
4. Normal mobs (zombies, skeletons, spiders) spawn inside the witch hut.