USkeletalMeshComponent::SkeletalMeshAsset modified value is lost when changing via level details panel

USkeletalMeshComponent::SkeletalMeshAsset is transient so it seems that any modified value that is set via the level details panel on an existing actor is lost.  We deal with this same problem for other transient values via FSkeletalMeshComponentInstanceData so should probably do the same for SkeletalMeshAsset

1. Create a new pawn blueprint
2. Add a skeletal mesh component
3. Set the skeletal mesh component in the blueprint editor
4. Drop the blueprint into the level viewport
5. Select the actor you just created and, via the details panel, change the Skeletal Mesh Asset property
   1. Expected behaviour: The mesh in the viewport should change and the asset referenced by Skeletal Mesh Asset in the details panel should update
   2. Actual behaviour: The mesh in the viewport changes but the asset referenced by Skeletal Mesh Asset does not