Durability decrease from Thorns enchantment doesn't remove item

To reproduce:

1. Equip a piece of Thorns armor with low durability:

/replaceitem entity @p slot.armor.chest diamond_chestplate 1 527 {ench:[{id:7s,lvl:1s}]}

2. Let a zombie hit you

Observe the stack size be set to 0. This piece of armor will never disappear from losing durability.

Originally found here: https://www.reddit.com/r/Minecraft/comments/4e6xzx/found_a_strange_bug_with_durability_on_my/d1xkz8a