Use GeoJsonSerializer when geojson is requested via the format query param

[SAY-5]

Changes / Features implemented

DataViewViewSet.data resolves the export format from the URL kwarg and falls back to the format query param, but get_serializer_class only looked at the kwarg. Requesting geojson via ?format=geojson therefore took the geojson branch in data while get_serializer_class returned JsonDataSerializer, which calls DECRYPTION_ERROR in instance on raw Instance objects and raises TypeError: argument of type 'Instance' is not iterable. This makes get_serializer_class use the same fallback so the GeoJsonSerializer is selected, and adds a regression assertion to the geojson dataview test.

Closes #3114.

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