Return instance of CraftInventoryBeacon for Beacons.  Fixes BUKKIT-4521

**The Issue:**
The implementation of InventoryHolder.getInventory() in CraftBeacon returns a new CraftInventory(tileentitybeacon). The BeaconInventory and CraftInventoryBeacon classes do exist within CraftBukkit but are never used, this appears to have been a hiccup when the CraftBeacon was first made.

**PR Breakdown:**
This commit changes the returned value for getHolder() to a CraftInventoryBeacon, which is what should be returned

**Jira Ticket:**
https://bukkit.atlassian.net/browse/BUKKIT-4521