Piwik\Tracker\Response::getOutput PHP Method

    public function getOutput()
    {
        $this->outputAccessControlHeaders();
        if (is_null($this->content) && ob_get_level() > 0) {
            $this->content = ob_get_clean();
        }
        return $this->content;
    }

 public function getOutput()
 {
     Common::sendHeader('Content-Type: application/json');
     return parent::getOutput();
 }