Fix detection of MIME type in message_parse()

Be careful, content-type may contain tricky content like in the
following example:

Content-Type: multipart/related;
  boundary="_004_3f1e4da175f349248b8d43cdeb9866f1AMSPR06MB343eurprd06pro_";
  type="text/html"

So test the MIME type with startswith()