Fixed Oracle GIS gml() test failure introduced by e910340; refs #18757.

@mjtamlyn, one more test failure related to the converters change. The results of [GeoQuerySet.gml()](https://github.com/django/django/blob/9d30412a5ad1c72b3d319b4c2bceacb53a0ff1da/django/contrib/gis/db/models/query.py#L168-L179v) aren't coerced to a string. `extra_select_fields.get('gml')` here doesn't return a field so the converters aren't run. I hope you can suggest a more elegant solution.