the warning message invisible again

[Kai Wang]

After Julian upgraded MailScanner to version 3.14, the warning
message invisible thing didn't bother us for a while. It showed up
again. Some of the infected messages with the content type like
the following were not converted to "multipart/mixed".

------------------------------------------------------------------
......
MIME-Version: 1.0
Content-Type: multipart/related;
         type="multipart/alternative";
         boundary="====_ABC1234567890DEF_===="
X-Priority: 3
X-MSMail-Priority: Normal
X-Unsent: 1
X-MailScanner: Found to be infected, Found to be clean
......
------------------------------------------------------------------

Kai