private function formatMessage(array $message)
{
if (isset($message['data']) && $this->encode) {
$message['data'] = base64_encode($message['data']);
}
if (!array_key_exists('data', $message) && !array_key_exists('attributes', $message)) {
throw new InvalidArgumentException('At least one of $data or
$attributes must be specified on each message, but neither
was given.');
}
return $message;
}