Fix handling of Message-Id: header.
Fix handling of Message-Id: header.

If there's no Message-Id:, we were returning without processing the
References: and In-Reply-To: headers.

Also, if there was a Message-Id: header but we couldn't make sense of it,
we were putting the md5sum of an empty string into the MessageInfo hash.

GNOME/evolution-ews,GNOME/evolution-ews