fix: remove default value for mailclass

The initial setup of green arrow has not have mail class defined, the default mailclass is implicit

Set the value with `default` will cause an error because the mailclass does not exist

GreenArrow.Engine/Model/Message.cs

@@ -75,7 +75,7 @@ public class Message
         /// <summary>
         /// Name of the SimpleMH Mail Class to use. This key has the same effect as the X-GreenArrow-MailClass header.
         /// </summary>
-        public string MailClass { get; set; } = "default";
+        public string MailClass { get; set; }
 
         /// <summary>
         /// Fully qualified domain name to use in the domain portion of the Message-ID header generated by GreenArrow.