Don't copy dragged inventory items, just re-wrap them.

I apologize for not having an existing Jira issue, but this is something I found while debugging an inventory issue involving custom data tags.

I've been testing with this change and it seems fine. It's probably wasteful to copy the items each time they're dragged, and it will also destroy any nbt data that isn't Bukkit-managed.

I know you don't actively support plugins modifying NBT data, but I'm hoping Bukkit will generally try to preserve data if it's there?

There certainly may be a reason this line of code is the way it is that I'm not aware of, in which case sorry- and I'd love to know what the reason is :) Thanks!