protected function postRequest($uri, $body = null, $options = array())
    {
        if ($this->testMode) {
            $options['debug'] = true;
        }

        $headers = array(
            'Content-Type'               => 'application/json',
            'Accept'                     => 'application/json',
            'X-Verkkomaksut-Api-Version' => $this->apiVersion,
        );

        $options = array_merge(
            array(
                'auth'            => array($this->_apiKey, $this->_apiSecret),
                'timeout'         => 30,
                'connect_timeout' => 10,
            ),
            $options
        );

        $request = new Request('POST', $uri, $headers, $body);

        return $this->_client->send($request, $options);
    }