Fix automatic header calculation

[ageevvalentin]

After update my project to iOS 15 I found strange problem. I have a view controller with FSCalendar which fill it and open this view controller with a pageSheet modal presentation. When I open it above an other controller with fullScreen modal presentation (it not reproduced with a pageSheet modal presentation), I see a header which intersects other elements. I debugged header size, it was -1. I found strange condition in preferredHeaderHeight(). I think need to use _preferredHeaderHeight instead of _preferredWeekdayHeight. _preferredWeekdayHeight already used and set into preferredWeekdayHeight()