private function setDefaultHeaders()
    {
        $defaultHeaders = [
            'Content-Type' => 'application/json',
            'Accept' => 'application/json',
            'User-Agent' =>  Client::UA,
            'X-MOLTIN-SDK-LANGUAGE' => 'php',
            'X-MOLTIN-SDK-VERSION' => 'v2-dev'
        ];

        foreach($defaultHeaders as $name => $value) {
            if (!$this->getHeader($name)) {
                $this->addHeader($name, $value);
            }
        }

        return $this;
    }