Support for null envelope sender

I'm using Mail to generate Delivery Status Notifications. It's required that these messages have a null envelope sender. That is, "<>". See line 433 of RFC 3464 (added to the references directory) for a reference. Note, also, that a null envelope sender is recommended for regular auto-response mail, like vacation auto responders and such, to help prevent mail loops.

So, here's a patch to support that behavior in the Mail gem.
This Element is parsing the informational header on delivered messages. It has a confusingly similar name, but it's a whole separate concern from delivery. A message that's delivered with a `<>` envelope-from will still have delivery date and such since this is written by the receiving MTA.