Returns a HMAC-SHA-1 signature for the given query string.
        http://goo.gl/R4O0E

def generate_message_signature(self, query_string):
        """
        Returns a HMAC-SHA-1 signature for the given query string.
        http://goo.gl/R4O0E
        """
        # split for sorting
        pairs = query_string.split('&')
        pairs = [pair.split('=', 1) for pair in pairs]
        pairs_sorted = sorted(pairs)
        pairs_string = '&' . join(['=' . join(pair) for pair in pairs_sorted])

        digest = hmac.new(self.key, b(pairs_string), hashlib.sha1).digest()
        signature = base64.b64encode(digest).decode('utf-8')

        return signature