Use appropriate type when recreating routes for nested_routes

Commit ca7ba07b4e42bd1c7c6bb8088c0c5a2c434b56ee to the rest framework adds new Route types. This takes that into account when rebuilding routes for the NestedSimpleRouter.

https://github.com/tomchristie/django-rest-framework/blob/ca7ba07b4e42bd1c7c6bb8088c0c5a2c434b56ee/rest_framework/routers.py