[Xwt.Mac] Add in an explicit cast to convert DateTime to NSDate
[Xwt.Mac] Add in an explicit cast to convert DateTime to NSDate

Newer XamMac will require this as the API has changed. There is
no longer an implicit cast from DateTime to NSDate, only an explicit
one.

mono/xwt,steffenWi/xwt,mminns/xwt,lytico/xwt,mminns/xwt,antmicro/xwt,TheBrainTech/xwt,cra0zy/xwt,hamekoz/xwt,akrisiun/xwt,residuum/xwt,hwthomas/xwt,iainx/xwt,directhex/xwt