In my survival world in Minecraft 1.9 snapshots I built an XP farm with a naturally generated skeleton spawner. The spawner didn't spawn anything. I think it is a bug, because as you can see on this video (https://www.youtube.com/watch?v=fI1nVRsBNfo&feature=youtu.be), there is no light in the spawner room and I watched some videos from Minecraft 1.8 and the same XP farm worked (spawner was spawning mobs).

Sorry if it is not a bug, but if it is, thank you for fixing