Use `path_info` instead of `path`

`RelatedField.should_full_dehydrate` uses `bundle.request.path`. This might be changed to `path_info`, which is required under some web server configs, for example when _Django_ is deployed under a subpath (e.g. `/myproject` instead of `/`; [see the docs](https://docs.djangoproject.com/en/dev/ref/request-response/#django.http.HttpRequest.path_info)).