mail.message.Message has case-insensitive behavior for headers, but setting the header value *adds* a new header even if the same header already exists. This caused multiple issues when forwarding, because the forwarded mail kept the original To: header. Also changed the code to use capitalized header names at all times, just for consistency. Also removed unused variable ``mail_message``. bzr revid: odo@openerp.com-20120427140427-3qfctfqzrsexgkac