/replaceitem cannot place non-armor items into Armor Stands' head slot

UPDATE: As of 15w32a this bug only exists for Armor Stands. All other mobs and players appear to work.

Any item can still be put on a player's head via replaceitem, but mobs can only wear helmets. This appears unintentional as per https://twitter.com/SeargeDP/status/623125935445921792

Works:
/replaceitem entity @e[type=ArmorStand] slot.armor.head minecraft:diamond_helmet

Does not work:
/replaceitem entity @e[type=ArmorStand] slot.armor.head minecraft:apple