Rename our post meta for the order number to not contain spaces.

Apparently, sorting by a post meta will not work if the meta contains spaces...
It seems to be a convention to start all post metas with an underscore and separate each word by an underscore, too...

woocommerce-advanced-ordernumbers.php

@@ -31,7 +31,7 @@ if ( in_array( 'woocommerce/woocommerce.php', apply_filters( 'active_plugins', g
 
 	if (!class_exists("OpenToolsOrdernumbers")) {
 		class OpenToolsOrdernumbers {
-			public $ordernumber_meta = "Order Number";
+			public $ordernumber_meta = "_order_number";
 			
 			/**
 			 * Construct the plugin object