Mob Spawner spawns entities under any light level.

Mob spawners are now spawning zombies, spiders, etc. under any light level condition. To test this, you can start a flat map and place a spawner:

Place a Skeleton Spawner

/setblock ~ ~1 ~ mob_spawner 0 replace {EntityId:Skeleton}

This will place the spawner at your head height. Nothing will spawn – until you place at least one block down on the ground within the spawner's 8 by 8 spawning grid. Then, even if it is the middle of the day under full light, an entity will spawn within the block that is placed. The block can be placed on the ground, at the spawner level, or one level above the spawner within the 8x8 grid to see this.