Firestore: 'Query.order_by("__name__")' implies converting field values to extended paths.

Per the `query-cursor-vals-docid` conformance test.  

```python
query = collection.order_by('__name__').start_at(['a']).end_at(['z'])
```

The  `'a'` should be converted to `/projects/<project-id>/databases/<database>/<collection_path>/a`;  likewise the `z`.