Capture response headers by default

[AnrDaemon]

Install default internal CURLOPT_HEADERFUNCTION handler.
Override setting of CURLOPT_HEADERFUNCTION option.
Integrate external callback into internal handler.

class Browser {
public function writeHeader($curl, $header) {
  $this->header .= $header;
  // Return count from external handler to fail in case of its failure
  return call_user_func($this->handler, null, $header); 
}

[AnrDaemon]

Almost done in dev.