[FIX] mail: prepare mail header References to avoid line breaks

When Odoo routes incoming emails it is looking for existing messages in
database using Message Ids which are coming from e-mail header
References.

In Odoo the message id looks pretty long like

    743570479975566.1584086032.522504091262817-openerp-message-notify@ip-172-31-45-160

As it declared in [RFC2822] long header bodies can be "folded" using
CRLF+WSP. And some mail clients do that very thing. They split
References header body which contains Message Ids by "\n ".  The example
of mail client where it can be reproduced is apps.rackspace.com We
created Sales Order in Odoo, sent this quotation to the client email. He
replied with e-mail, and this email can't be matched with any existing
message id and as result it's not attached to the Sales Order.

RFC2882: https://tools.ietf.org/html/rfc2822#section-2.2.3



closes odoo/odoo#67991

Signed-off-by: Julien Castiaux <Julien00859@users.noreply.github.com>