Clone ItemStack before adding to player inventory.

Spigot is at it again with another breaking change to the API where the
JavaDocs have been retrofitted to match the new, ancient way of writing
software.

The addItem and removeItem methods originally just added or removed
whatever they could when called, and returned a map of items that failed
due to lack of space/items. Now, with the breaking API change, these
methods actually *mutate their arguments* - a practice that has been
heavily discouraged for decades in modern software development, and
something you usually only see in C these days. This change, like many
others in the same vein, show the complete recklessness, incompetence,
and lack of awareness from an otherwise talented team of developers.
Hopefully, they will continue to hone their skills so that we can
eventually have a stable API again.

src/main/java/com/garbagemule/MobArena/things/ItemStackThing.java

@@ -13,12 +13,12 @@ public class ItemStackThing implements Thing {
 
     @Override
     public boolean giveTo(Player player) {
-        return player.getInventory().addItem(stack).isEmpty();
+        return player.getInventory().addItem(stack.clone()).isEmpty();
     }
 
     @Override
     public boolean takeFrom(Player player) {
-        return player.getInventory().removeItem(stack).isEmpty();
+        return player.getInventory().removeItem(stack.clone()).isEmpty();
     }
 
     @Override