public function setAuth($user, $password = '', $scheme = self::AUTH_BASIC)
    {
        if (empty($user)) {
            $this->auth = null;
        } else {
            $this->auth = array(
                'user'     => (string)$user,
                'password' => (string)$password,
                'scheme'   => $scheme
            );
        }

        return $this;
    }