public function setRequestBody($string) {
        if (!empty($this->requestOptions['multipart']) || !empty($this->requestOptions['form_params'])) {
            throw new InvalidArgumentException(
                'It\'s not allowed to set a request body when using multipart/form-data or form parameters.'
            );
        }

        $this->request = $this->request->withBody(Psr7\stream_for($string));

        return $this;
    }