Fix LeadedChest does not drop items.

Signed-off-by: yushijinhun <yushijinhun@gmail.com>
Fix it throws an exception when add an upgrade to a polytool.

Signed-off-by: yushijinhun <yushijinhun@gmail.com>
Reuse ItemStack object.

Signed-off-by: yushijinhun <yushijinhun@gmail.com>
Merge pull request #204 from yushijinhun/fix-bugs

Fix bugs