Request a discharge macaroon from the caveat location
        as an HTTP URL.
        @param cav Third party {pymacaroons.Caveat} to be discharged.
        @param payload External caveat data {bytes}.
        @return The acquired macaroon {macaroonbakery.Macaroon}

def acquire_discharge(self, cav, payload):
        ''' Request a discharge macaroon from the caveat location
        as an HTTP URL.
        @param cav Third party {pymacaroons.Caveat} to be discharged.
        @param payload External caveat data {bytes}.
        @return The acquired macaroon {macaroonbakery.Macaroon}
        '''
        resp = self._acquire_discharge_with_token(cav, payload, None)
        # TODO Fabrice what is the other http response possible ??
        if resp.status_code == 200:
            return bakery.Macaroon.from_dict(resp.json().get('Macaroon'))
        cause = Error.from_dict(resp.json())
        if cause.code != ERR_INTERACTION_REQUIRED:
            raise DischargeError(cause.message)
        if cause.info is None:
            raise DischargeError(
                'interaction-required response with no info: {}'.format(
                    resp.json())
            )
        loc = cav.location
        if not loc.endswith('/'):
            loc = loc + '/'
        token, m = self._interact(loc, cause, payload)
        if m is not None:
            # We've acquired the macaroon directly via legacy interaction.
            return m
        # Try to acquire the discharge again, but this time with
        # the token acquired by the interaction method.
        resp = self._acquire_discharge_with_token(cav, payload, token)
        if resp.status_code == 200:
            return bakery.Macaroon.from_dict(resp.json().get('Macaroon'))
        else:
            raise DischargeError(
                'discharge failed with code {}'.format(resp.status_code))