Added pop_call method on rpc protocol

        err = {'exception': error, 'message': message}

        self._send_response(msg_id, error=err)

    def pop_call(self, msg_id, default=None):

        '''

        Pop a call from the expected call dict.

        :param msg_id: id of the call to pop

        :param default: value to get if no call is found with the provided id

        '''

        return self._calls.pop(msg_id, default)

    def call(self, method_name, *args, **kwargs):

        '''

        Make a new remote call on the peer.