Fix Shake poisoning the treasure cache with Wither skeletons

The returned ItemStack is modified for both Sheep and Skeletons, so a copy should be returned.

src/main/java/com/gmail/nossr50/skills/fishing/Fishing.java

@@ -145,7 +145,7 @@ public final class Fishing {
             cumulatedProbability += treasure.getDropChance();
 
             if (dropProbability < cumulatedProbability) {
-                return treasure.getDrop();
+                return treasure.getDrop().clone();
             }
         }