Hoppers Can Insert & Extract From Reinforced Double Chests

[gaffy00]

Hoppers can extract items from, and insert items into reinforced double chests, without the chest being made insecure.

Note that it is specifically double chests that are vulnerable, single chests are unaffected.

Reason: The onInventoryMoveItemEvent in InventoryListener checks if source and destination blocks are instances of Container, but double chests only implement InventoryHolder.

PR with fix to follow.