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...
Intended.
While not an official source, the MC Wiki states that "mushrooms grow only at light level 12 and below."