Allow plugins to enchant already enchanted items. Fixes BUKKIT-1956

This causes the server to generate PrepareItemEnchantEvent even in the
case that an item is already enchanted or otherwise would normally not
be enchantable.

Remapped by Srg2Source from Bukkit/CraftBukkit@5a018f26ca69d0702426c7113dd1d63ed76b2e15