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.
Trouble with removing this is that someone who doesn't set a from address on the message will end up with a `<>` envelope from.

Rather than doing `nil` <-> `<>` translation, it'd be clearer to set the envelope-from to literal `<>` and keep this check in place.