Fixed: minecraft doesn't handle the trade if the item on the cursor
already has full stack size, however previously we still did run the
code for adding and removing the traded items from the player's shop
chest, even though no trade has actually taken place.
We now check for this specific situation and don't trigger our trading
code if the item on the cursor already has full stack size. Also added a
debug message for this case, so you can see what is going on in debug
mode.