WitherSkulls instantly revert back to a Rotation of 90, -90 after having their Rotation altered.

Steps to reproduce:

/summon WitherSkull ~ ~ ~

{direction:[],CustomName:"Test1",CustomNameDisplay:1}

/entitydata @e[name=Test1]

{Rotation:[0f,0f]}

#Says that Rotation was set to 0, 0

/entitydata @e[name=Test1]

{CustomNameVisible:1}

#Says that Rotation is still 90, -90