bounce: Handle rfc/1892 style bounce notices

Sometimes an MTA may just send back the headers of the original message
rather than the entire original message. Such an email will have a
`text/rfc822-headers` part which will be the complete headers of the
original message.

Without this patch, osTicket will create a new ticket for the bounce message
rather than attaching the new internal note to the existing ticket.

References:
https://tools.ietf.org/html/rfc1892