Fixed a VERY long standing bug discovered by libraryaddict.

Constructing a WrappedWatchableObject with a 
net.minecraft.server.ItemStack would cause ProtocolLib to throw an
IllegalArgumentException, even though WrappedWatchableObject.
setValue(Object) accepts ItemStacks without trouble.

This is because WrappedWatchableObject.getUnwrappedType() didn't handle 
ItemStacks at all.

ProtocolLib/src/main/java/com/comphenix/protocol/wrappers/WrappedWatchableObject.java

@@ -346,6 +346,8 @@ public class WrappedWatchableObject extends AbstractWrapper {
 			return MinecraftReflection.getChunkPositionClass(); 
 		else if (wrapped.equals(WrappedChunkCoordinate.class))
 			return MinecraftReflection.getChunkCoordinatesClass();
+		else if (ItemStack.class.isAssignableFrom(wrapped))
+			return MinecraftReflection.getItemStackClass();
 		else
 			return wrapped;
 	}

ProtocolLib/src/test/java/com/comphenix/protocol/wrappers/WrappedWatchableObjectTest.java

@@ -0,0 +1,31 @@
+package com.comphenix.protocol.wrappers;
+
+import static org.junit.Assert.*;
+
+import org.bukkit.Material;
+import org.bukkit.craftbukkit.v1_7_R1.inventory.CraftItemFactory;
+import org.bukkit.inventory.ItemStack;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.powermock.core.classloader.annotations.PrepareForTest;
+
+import com.comphenix.protocol.BukkitInitialization;
+
+@RunWith(org.powermock.modules.junit4.PowerMockRunner.class)
+@PrepareForTest(CraftItemFactory.class)
+public class WrappedWatchableObjectTest {
+	@BeforeClass
+	public static void initializeBukkit() throws IllegalAccessException {
+		BukkitInitialization.initializeItemMeta();
+	}
+	
+	@Test
+	public void testItemStack() {
+		final ItemStack stack = new ItemStack(Material.GOLD_AXE);
+		final WrappedWatchableObject test = new WrappedWatchableObject(0, stack);
+	
+		ItemStack value = (ItemStack) test.getValue();
+		assertEquals(value.getType(), stack.getType());
+	}
+}