Don't filter item data for anvils. Fixes BUKKIT-2640

Filtering item data is usually a good idea to make sure we don't have
invalid data or data on items that shouldn't have it. However, anvils
use item data in slightly different way and so running its code for
filtering here causes the data to be corrupted.

src/main/java/net/minecraft/server/ItemStack.java

@@ -154,7 +154,7 @@ public final class ItemStack {
     }
 
     public void setData(int i) {
-        this.damage = (this.id > 0) && (this.id < 256) ? Item.byId[this.id].filterData(i) : i; // CraftBukkit
+        this.damage = (this.id > 0) && (this.id < 256) && (this.id != Block.ANVIL.id) ? Item.byId[this.id].filterData(i) : i; // CraftBukkit
     }
 
     public int k() {