Update off hand items when closing a gui

Due to what appears to be a client issue, the client rejects off hand modifications from the server when an inventory is opened, which causes #95. To slightly mitigate this in the mean time, the off hand will be updated when a gui is closed.

IF/src/main/java/com/github/stefvanschie/inventoryframework/gui/GuiListener.java

@@ -294,8 +294,14 @@ public void onInventoryClose(@NotNull InventoryCloseEvent event) {
             return;
         }
 
+        HumanEntity humanEntity = event.getPlayer();
+        PlayerInventory playerInventory = humanEntity.getInventory();
+
+        //due to a client issue off-hand items appear as ghost items, this updates the off-hand correctly client-side
+        playerInventory.setItemInOffHand(playerInventory.getItemInOffHand());
+
         if (gui.isUpdating()) {
-            gui.getHumanEntityCache().restoreAndForget(event.getPlayer());
+            gui.getHumanEntityCache().restoreAndForget(humanEntity);
 
             if (gui.getViewerCount() == 1) {
                 activeGuiInstances.remove(gui);
@@ -305,8 +311,6 @@ public void onInventoryClose(@NotNull InventoryCloseEvent event) {
 
             //this is a hack to remove items correctly when players press the x button in a beacon
             Bukkit.getScheduler().runTask(JavaPlugin.getProvidingPlugin(getClass()), () -> {
-                HumanEntity humanEntity = event.getPlayer();
-
                 if (humanEntity.getOpenInventory().getTopInventory() instanceof PlayerInventory) {
                     humanEntity.closeInventory();
                 }