encryption/signing should be last thing
move setting some headers to ::Interface::Email

We were setting MIME-Version and Content-Transfer-Encoding
in Action::SendEmail. This code has some history and we should
keep it. These days we have more API to send emails besides
SendEmail scrip action, so it's good to move this code into
place which handles all emails.

See http://issues.bestpractical.com/Ticket/Display.html?id=16818
Merge branch '4.2/refactor-send-mail-code'