If a username and password are present - attempt to use them to
        request a fresh SAS token.

def update_token(self):
        """If a username and password are present - attempt to use them to
        request a fresh SAS token.
        """
        if not self.username or not self.password:
            raise errors.TokenExpired("Unable to refresh token - no username or password.")
        encoded_uri = compat.quote_plus(self.uri).encode(self._encoding)  # pylint: disable=no-member
        encoded_key = compat.quote_plus(self.username).encode(self._encoding)  # pylint: disable=no-member
        self.expires_at = time.time() + self.expires_in.seconds
        self.token = utils.create_sas_token(
            encoded_key,
            self.password.encode(self._encoding),
            encoded_uri,
            self.expires_in)