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.
@bf4 the spec had the same issue as others have had, stubbing Time when the field actually uses DateTime: https://github.com/mikel/mail/blob/master/lib/mail/fields/resent_date_field.rb#L17-L20