SPIGOT-4646: Test + fix InventoryWrapper.getContents

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/InventoryWrapper.java

@@ -128,7 +128,7 @@ public class InventoryWrapper implements IInventory {
         List<ItemStack> items = new ArrayList<ItemStack>(size);
 
         for (int i = 0; i < size; i++) {
-            items.set(i, getItem(i));
+            items.add(getItem(i));
         }
 
         return items;

paper-server/src/test/java/org/bukkit/craftbukkit/inventory/InventoryWrapperTest.java

@@ -0,0 +1,23 @@
+package org.bukkit.craftbukkit.inventory;
+
+import junit.framework.Assert;
+import org.bukkit.Material;
+import org.bukkit.inventory.Inventory;
+import org.bukkit.inventory.ItemStack;
+import org.junit.Test;
+
+public class InventoryWrapperTest {
+
+    @Test
+    public void test() {
+        Inventory raw = new CraftInventoryCustom(null, 9);
+        raw.addItem(new ItemStack(Material.STONE));
+
+        Assert.assertTrue(raw.contains(Material.STONE));
+
+        InventoryWrapper wrapper = new InventoryWrapper(raw);
+        CraftInventory proxy = new CraftInventory(wrapper);
+
+        Assert.assertTrue(proxy.contains(Material.STONE));
+    }
+}