pipe: Unwrap forwarded emails

If the mail received into the system has a content type of 'message/rfc822',
then unwrap the inner message and use it as the parsed email. Effectively,
assume the email was forwarded as an message/rfc822 attachment.
Honor inline bodies as valid message

Previously, if a message had a Content-Disposition header, then it could not
be considered as the body of the message.