Bug in circuit create serializer: Doesn't include the "site" field.

This isn't caught because we have few use-cases that include top-level objects that aren't using site-specific API endpoints. We are not testing for this either.

e.g. `POST /api/circuits/` is hardly tested if at all, while `POST /api/sites/1/circuits/` is the bulk of our testing. I'm pretty certain that a test in the former case would fail with an error about missing `site` (`site_id`) since this value passed in implicitly in the latter example (more common).

So this isn't an active bug, but it's one I noted while working on the `Protocol` feature.