Test dump of null geometry features

  >>> import geojson 
  >>> geojson.dumps(geojson.Feature(id=12, geometry=None, properties={'foo': 'bar'})) # doctest: +ELLIPSIS 
  '{"geometry": null, "type": "Feature", "properties": {"foo": "bar"}, "id": 12}' 
  >>> geojson.dumps(geojson.Feature(id=12, properties={'foo': 'bar'})) # doctest: +ELLIPSIS 
  '{"geometry": null, "type": "Feature", "properties": {"foo": "bar"}, "id": 12}'