Fix on_disconnect callback handling.

    :param connection: the connection serving this :class:`RpcProtocol`

    :param label: the label of this :class:`RpcProtocol` instance

    :param handler: command handler to bind by default

    :param on_disconnect: callback called when client disconnect

    :param on_disconnect: callback called when client disconnect, could be the

        name of an RPC handler or a callable

    :param timeout: global command timeout

    '''

            self._handle_response({'id': cid, 'return': None, 'error': err})

        # Execute on_disconnect callback:

        if self._on_disconnect is None:

            return

        callback = None

        if self._on_disconnect is not None and not callable(self._on_disconnect):

        if not callable(self._on_disconnect):

            if self._handler is not None:

                try:

                    callback = self._handler[self._on_disconnect]

                self.logger.warn('Shutdown callback specified but no handler '

                                 'binded on rpc, ignoring')

                callback = None

        else:

            callback = self._on_disconnect

        if callback is not None:

            try: