Spawners are not spawning monsters anymore

The bug

Spawners do not spawn monsters anymore even if the required conditions are met.

How to reproduce

1. Set the difficulty to something different than "Peaceful" (/difficulty easy)
2. Set the time to night /time set night
3. Use the following command

   /setblock ~ ~ ~ mob_spawner 0 replace {SpawnData:{id:"Zombie"}}
   

Note: This worked for 1.9-pre1