BugFix: Making sure that the start or end date comes from the url.

This is to make sure that a cookie is not overriding the value
of one of those date.

Maybe we should start discussing what other var ought to be
protected that way. I did only those, because I was reasonably
confident that it was the purpose.

The bug was explained there. https://g4j.digital-peak.com/support/forum/11-com-gcalendar/17820-gcalendar-problem/

To synthesize the bug fix, we are now sure that those value are not overridden by a cookie value.