invalid iCal message if SEQUENCE not set

[m-a-v]

https://github.com/sabre-io/vobject/blob/d529dda7ad8d80cf171145d767726fd9f90bf3e0/lib/ITip/Broker.php#L493

I think this should be changed to something like.

$message->sequence = isset($eventInfo['sequence']) ? $eventInfo['sequence'] : 0;

Otherwise the iCal message will be invalid.