I used this command in a command block:

/summon Item 377 66 297 {Item:{id:minecraft:iron_axe,Count:1,tag:{display:{Name:"Decapitator"},CanDestroy:["minecraft:skull"]}},Age:-32768}

, and in the NBT tags in the bottom of the item tooltip, instead of:

May Break:
Skull

I saw:

May Break:
tile.skull.skeleton.name

This bug may persist in the recent 1.9 snapshots as I haven't tested it, nor have I tested it with other NBT tags.