Fix test at test_geometry_serializer_method_field_nogeo (#2)

The function `add_to_fields` does not have any checks for `None` and
since the app now supports `geo_field = None` it adds it to the fields
of the serializer. In any case, adding an `assert` in the `add_to_fields`
function would have made the error obvious in the first place.

rest_framework_gis/serializers.py

@@ -96,7 +96,9 @@ def add_to_fields(field_name):
  meta.fields += additional_fields
 
  check_excludes(meta.geo_field, 'geo_field')
- add_to_fields(meta.geo_field)
+
+ if meta.geo_field is not None:
+ add_to_fields(meta.geo_field)
 
  meta.bbox_geo_field = getattr(meta, 'bbox_geo_field', None)
  if meta.bbox_geo_field: