I summoned a wither skeleton (I have a redstone invention that gives them fire resistance [to zombies and skeletons] to prevent annoying sounds when they get hurt) with /summon Skeleton ~ ~ ~ {SkeletonType:1} and it put a wither skeleton down and it flashes in sunlight. I know it has Fire Resistance but it still should not burn, since nether mobs have fire resistance naturally, and it is sporadically lit on fire for one frame.
Environment
Windows 8
Wither Skeletons do not burn in sunlight. Their "fire" animation is buggy and prevents them from burning.