/**
* Enable response chunking
*
* @link https://github.com/chriso/klein.php/wiki/Response-Chunking
* @link http://bit.ly/hg3gHb
* @param string $str An optional string to send as a response "chunk"
* @access public
* @return Response
*/
public function chunk($str = null)
{
parent::chunk();
if (null !== $str) {
printf("%x\r\n", strlen($str));
echo "{$str}\r\n";
flush();
}
return $this;
}