def authorization_url(self, **kwargs):
        """
        Get authorization URL to redirect the resource owner to.

        https://tools.ietf.org/html/rfc6749#section-4.1.1

        :param str redirect_uri: (optional) Absolute URL of the client where
            the user-agent will be redirected to.
        :param str scope: (optional) Space delimited list of strings.
        :param str state: (optional) An opaque value used by the client to
            maintain state between the request and callback
        :return: URL to redirect the resource owner to
        :rtype: str
        """
        payload = {'response_type': 'code', 'client_id': self._client_id}

        for key in kwargs.keys():
            # Add items in a sorted way for unittest purposes.
            payload[key] = kwargs[key]
        payload = sorted(payload.items(), key=lambda val: val[0])
        params = urlencode(payload)
        url = self.get_url('authorization_endpoint')

        return '{}?{}'.format(url, params)