Copying a world will reset chests

Put the summary of the bug you're having here

What I expected to happen was...:
I expected that items in chests would stay

What actually happened was...:
I copies my friend's world, and all of the items in my chests were gone!

Steps to Reproduce:
1. Make a new world.
2. Invite your Friend.
3. Tell him/her to put some unvaluable items in a chest.
4. Tell him/her to copy the world.
5. The items in the chest is gone!