Spawners spawn mobs in blocks even with light

Put the summary of the bug you're having here

What I expected to happen was...: Mobs not to spawn from a lit spawner so I could create a mob farm
Describe what you thought should happen here

What actually happened was...:Mobs spawned in the blocks and jumped out to attack me
Describe what happened here

Steps to Reproduce:
1. Find a dungeon
2. Cover spawner in torches
3. Watch as the mobs spawn in nearby blocks and jump out to assault your face