Restore custom InventoryHolder support

Upstream removed the ability to consistently use a custom InventoryHolder,
However, the implementation does not use an InventoryHolder in any form
outside of custom inventories.

We can take that knowledge and apply some expected behavior, if we're given
an inventory holder, we should use it and return a custom inventory with the
holder, otherwise, create an inventory backed by the intended inventory, as
per upstream behavior.

This provides a "best of both worlds" scenario: plugins with InventoryHolder's
will always work as intended in the past, those without will create implementation
based inventories.

Spigot-Server-Patches/0405-Restore-custom-InventoryHolder-support.patch

@@ -0,0 +1,46 @@
+From dd5f820cc342d58e451dd788cea6348718c6b2f0 Mon Sep 17 00:00:00 2001
+From: Shane Freeder <theboyetronic@gmail.com>
+Date: Mon, 5 Nov 2018 04:23:51 +0000
+Subject: [PATCH] Restore custom InventoryHolder support
+
+Upstream removed the ability to consistently use a custom InventoryHolder,
+However, the implementation does not use an InventoryHolder in any form
+outside of custom inventories.
+
+We can take that knowledge and apply some expected behavior, if we're given
+an inventory holder, we should use it and return a custom inventory with the
+holder, otherwise, create an inventory backed by the intended inventory, as
+per upstream behavior.
+
+This provides a "best of both worlds" scenario: plugins with InventoryHolder's
+will always work as intended in the past, those without will create implementation
+based inventories.
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/inventory/util/CraftInventoryCreator.java b/src/main/java/org/bukkit/craftbukkit/inventory/util/CraftInventoryCreator.java
+index 23389743d5..d25928e094 100644
+--- a/src/main/java/org/bukkit/craftbukkit/inventory/util/CraftInventoryCreator.java
++++ b/src/main/java/org/bukkit/craftbukkit/inventory/util/CraftInventoryCreator.java
+@@ -31,10 +31,20 @@ public final class CraftInventoryCreator {
+     }
+ 
+     public Inventory createInventory(InventoryHolder holder, InventoryType type) {
++        // Paper start
++        if (holder != null) {
++            return DEFAULT_CONVERTER.createInventory(holder, type);
++        }
++        //noinspection ConstantConditions // Paper end
+         return converterMap.get(type).createInventory(holder, type);
+     }
+ 
+     public Inventory createInventory(InventoryHolder holder, InventoryType type, String title) {
++        // Paper start
++        if (holder != null) {
++            return DEFAULT_CONVERTER.createInventory(holder, type, title);
++        }
++        //noinspection ConstantConditions // Paper end
+         return converterMap.get(type).createInventory(holder, type, title);
+     }
+ 
+-- 
+2.19.1
+