Returns a Premier account signed url. Docs on signature:
            https://developers.google.com/maps/documentation/business/webservices/auth#digital_signatures

def _get_signed_url(self, params):
        """
        Returns a Premier account signed url. Docs on signature:
            https://developers.google.com/maps/documentation/business/webservices/auth#digital_signatures
        """
        params['client'] = self.client_id

        if self.channel:
            params['channel'] = self.channel

        path = "?".join((self.api_path, urlencode(params)))
        signature = hmac.new(
            base64.urlsafe_b64decode(self.secret_key),
            path.encode('utf-8'),
            hashlib.sha1
        )
        signature = base64.urlsafe_b64encode(
            signature.digest()
        ).decode('utf-8')
        return '%s://%s%s&signature=%s' % (
            self.scheme, self.domain, path, signature
        )