// var headers = parse_headers( http.responseHeaders );
// headers.content_type.match( /^image/i )
function parse_headers( raw )
{
  var headers = {}, lines = raw.split( /[\r\n]+/ ), i, m, name;
  for( i in lines )
  {
    line = lines[i];
    if( m = lines[i].match( /^(^\S+)\s*:\s*(.*)$/ ) )
    {
      name = m[1].toLowerCase().replace(/-/g,'_');
      headers[name] = m[2]; // (last header wins)
    }
  }
  return headers;
}