Remove nested associations before adding new ones, to avoid collisions

E.g., say you have a `unique_together` set of fields `('parent', 'slug')`, and a model that initially serializes to this data:

    [
        {
            "id": 1,
            "parent": 1,
            "slug": "first",
            "text": "This is the initial version of the first element"
        },
        {
            "id": 2,
            "parent": 1,
            "slug": "second",
            "text": "This is the second element"
        }
    ]

Then in your client application you delete the first element and replace it with a new one. The data that you pass up to the API will be something like:

    [
        {
            "parent": 1,
            "slug": "first",
            "text": "This is a new version of the first element"
        },
        {
            "id": 2,
            "parent": 1,
            "slug": "second",
            "text": "This is the second element"
        }
    ]

This should be valid, but will cause an integrity error when deserialized because it will try to create the new element before deleting the old one.