Check correct bucket while milking cows.

Fixes WORLDGUARD-3508.

src/main/java/com/sk89q/worldguard/bukkit/listener/EventAbstractionListener.java

@@ -451,7 +451,7 @@ public void onPlayerBucketFill(PlayerBucketFillEvent event) {
         boolean allowed = false;
 
         // Milk buckets can't be emptied as of writing
-        if (event.getBucket() == Material.MILK_BUCKET) {
+        if (event.getItemStack().getType() == Material.MILK_BUCKET) {
             allowed = true;
         }