Fix NPC name in new mail notification when receiving mails from NPCs

If a player received a mail from a NPC with the same id as an existing character's guid, the mail notification would show the player's name instead.

src/game/Mails/MailHandler.cpp

@@ -824,20 +824,22 @@ void WorldSession::HandleQueryNextMailTime(WorldPacket& /**recv_data*/)
             if (now < m->deliver_time)
                 continue;
 
-            data << ObjectGuid(HIGHGUID_PLAYER, m->sender); // sender guid
+            HighGuid guidType = HIGHGUID_PLAYER;
 
             switch (m->messageType)
             {
-                case MAIL_AUCTION:
-                    data << uint32(m->sender);              // auction house id
-                    data << uint32(MAIL_AUCTION);           // message type
+                case MAIL_CREATURE:
+                    guidType = HIGHGUID_UNIT;
                     break;
-                default:
-                    data << uint32(0);
-                    data << uint32(0);
+                case MAIL_GAMEOBJECT:
+                    guidType = HIGHGUID_GAMEOBJECT;
                     break;
             }
 
+            data << ObjectGuid(guidType, m->sender);     // sender guid
+            data << static_cast<uint32>(m->sender);      // sender id
+            data << static_cast<uint32>(m->messageType); // message type
+
             data << uint32(m->stationery);
             data << uint32(0xC6000000);                     // float unk, time or something