Coalesce for F()
Updating fields that allow for NULLs must take care about those NULLs. Use case: `Message.objects.filter(id=1).update(price=CF('price', 0) + 7)` `Message.objects.filter(id=1).update(status=CF('status') // 's')` Works for postgres. Supposedly works for mysql. Didn't test with other backends.
- coalesce