reserach on handling of attributes (qdate,qdatetime) so serialization does not give problems on export

[dieuska]

We notice that storing qgis date attributes in the dict_properties gives problems when exporting it to geojson (qdate cannot be serialized). Research that this will be handled generic. At the moment this is fixed by loading these attributes as datetime and default serialisation is set to string so all objects can be serialized. check if this genercally the best way?

[koenedaele]

Dates are tricky as hell, especially with timezones and such. Consult @roefem for input on how we handle them in other places.