Date-field is not retained when adding original attributes

[dieuska]

At the moment , data-fields are transformed to string, to ensure json-loads/dumps. This has to be researched how we can handle this

[dieuska]

At the moment (after release brdrQ 0.9.8) all QDate-fields are transformed to datetime in preparation step. So the dates are handled as datetime in brdr. And there is no problem to export them back to QGIS. But maybe we should do further research if this can be handled more generally.
We also made a change in brdr (write_geojson) so objects that could not be serialized will be serialized as string by default so it does not give an error when exporting the results to geojson.