Creates a transaction for sending an asset.

        :param bytes asset_id: The ID of the asset being sent.
        :param TransferParameters transfer_spec: The parameters of the asset being transferred.
        :param bytes btc_change_script: The script where to send bitcoin change, if any.
        :param int fees: The fees to include in the transaction.
        :return: The resulting unsigned transaction.
        :rtype: CTransaction

def transfer_assets(self, asset_id, transfer_spec, btc_change_script, fees):
        """
        Creates a transaction for sending an asset.

        :param bytes asset_id: The ID of the asset being sent.
        :param TransferParameters transfer_spec: The parameters of the asset being transferred.
        :param bytes btc_change_script: The script where to send bitcoin change, if any.
        :param int fees: The fees to include in the transaction.
        :return: The resulting unsigned transaction.
        :rtype: CTransaction
        """
        return self.transfer(
            [(asset_id, transfer_spec)],
            TransferParameters(transfer_spec.unspent_outputs, None, btc_change_script, 0),
            fees)