I created a new world in 14w21b and walking around I found a mushroom biome. There are a lot of mushrooms!! I waited until the night passed and when it was full day, the mushrooms were ok, (not popin out due to light).

I post this because I noticed that in this version this bug might have been fixed: https://bugs.mojang.com/browse/MC-30877 I think the fix did not covered mushrooms or it might even worsened the situation...