IsBaby tag can't be changed from 1 to 0

To reproduce:

1. Summon a zombie
2. Use the command /entitydata @e[type=zombie,c=1] {IsBaby:1}
3. Watch as the zombie turns into a zombie
4. Use the command /entitydata @e[type=zombie,c=1] {IsBaby:0}
5. Watch as the zombie does not in fact turn into a non-baby
6. Use the command /entitydata @e[type=zombie,c=1] {Null:1}
7. Note that the value of 'IsBaby' is still listed as 0b.