Fixed ClassCastException in DungeonPack

Fixed a bug in DungeonPack that would trigger a rare ClassCastException
during dungeon selection. We were comparing instances of DungeonType
against WeightedContainer<DungeonType>. This wouldn't break dungeon
generation because DD falls back on selecting a random dungeon if
rule-based selection fails. It would allow duplicate dungeons to appear
in the same chain, though.