Test dump of null geometry features

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

  >>> geojson.dumps(geojson.Feature(id=12, properties={'foo': 'bar'}), sort_keys=True) # doctest: +ELLIPSIS,+NORMALIZE_WHITESPACE
  '{"geometry": null, "id": 12, "properties": {"foo": "bar"}, "type": "Feature"}'
