pipe: For wrapped messages, use wrapped headers

07fec4c4 introduced the ability to receive
and process messages forwarded as a message/rfc822. However, the email
headers that were saved on file in the %email_info_table were the headers of
the outer message.

This patch pulls the headers from the wrapped message to be saved in the
database. This mainly addresses the email References and In-Reply-To headers
for the email responses from the system.