::headers() update.

  1.  Close the message post-parse.
  2.  Decode the header-value.

  This means the following are equivilent:

    msg( header( "Subject" ) );

    msg( all_headers()["Subject"] );
  Downcase the keynames in the table returned by all_headers.

  This just makes things more pleasant.
  Write out a C-style string.

  Without this I get crashes via Glib convert-error, from
 CMessage::header().

  Seems like something is screwey, but not sure what.