A Wither Skeleton spawned with a spawner don't spawn holding a stone sword

When wither skeletons are spawned by an otherwise un-edited spawner, they don't by default spawn holding a stone sword.

What I was expecting to happen was:

The Wither Skeleton to spawn holding a stone sword.

What actually happened:

The Wither Skeleton spawned empty handed.

Steps to reproduce:

-Create a skeleton spawner with the SpawnData of SkeletonType: 1

-Watch it spawn a Wither Skeleton without a stone sword.