"set_nbt" function in loot table does not merge NBT to item

The bug

"function": "set_nbt" in loot tables does nothing. Loot tables that worked in 1.12 don't show an error when loaded, but when the item is dropped the NBT data is not applied to it. This causes, for example, uncraftable potions in fishing and uncraftable arrows dropped by strays.

Code analysis

It appears that in 17w48a's bka class, the NBT data from the item is merged to the NBT data from the table, but is not merged back onto the item.