Options requests

Some Django REST Framework methods expect a `get_object` method when they see that *PUT* requests are allowed (e.g., [`GenericAPIView.metadata`](https://github.com/tomchristie/django-rest-framework/blob/eab5933070d5df9078a6b88e85ee933cbfa28955/rest_framework/generics.py#L383)). For a bulk update view, the object being modified is the queryset itself, so this pull request returns that.

The `metadata` method is important because it's called during *OPTIONS* requests, which are necessary for CORS support.