Allow changing of format with query parameter

[sliverc]

Issue

The guides on https://django-rest-framework-json-api.readthedocs.io/en/stable/usage.html#configuration recommend including the rest_framework_json_api.filters.QueryParameterValidationFilter in initial configuration which is good advice, but by default, the Django REST Framework UI provides a dropdown which allows you to pick format=api or format=vnd.api+json

Without allowing the format parameter through you get the following error:

HTTP 400 Bad Request
Allow: GET, POST, HEAD, OPTIONS
Content-Type: application/vnd.api+json
Vary: Accept

{
    "errors": [
        {
            "detail": "invalid query parameter: format",
            "source": {
                "pointer": "/data"
            },
            "status": "400"
        }
    ]
}

What needs to change

• We document to change URL_FORMAT_OVERRIDE to contentFormat in our README.
• In the QueryParameterValidationFilter it's checked whether a query parameter is passed on which is equal configured value in URL_FORMAT_OVERRIDE. If this is true and the value of URL_FORMAT_OVERRIDE is set to something else than format then we let the user pass. If URL_FORMAT_OVERRIDE is still set to format a 400 error is raised (as would be the case today already without this change).

reference DRF docs

This issue is created out of the pull request #535