Double Chests don't work in DecayDungeons

[Ameliorate]

See #26 and 134dbf2. When you break half of a doublechest, dungeonmod will sometimes try to copy the whole inventoy of the doublechest into the singlechest, causing an exception. This is mitigated by not copying the inventory of a doublechest, however this is suboptimal because you can't use a doublechest in a dungeon now.