Patch request with "meta" attribute fails

[czosel]

(This is actually more an issue instead of a PR, but since I have a failing test, I figured I'd post this as a PR anyway.)

When meta is used as attribute name, PATCH requests are failing because the attribute's value is used in a code path which is supposed to handle the top-level meta property.

Request body (taken from the test case):

        request_data = {
            'data': {
                'type': 'comments',
                'id': self.second_comment.id,
                'attributes': {
                    'meta': "test"
                }
            }
        }

Exception:

Traceback (most recent call last):
  File "/home/christian/.pyenv/versions/3.6.7/envs/drf-json-api/lib/python3.6/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/home/christian/.pyenv/versions/3.6.7/envs/drf-json-api/lib/python3.6/site-packages/django/core/handlers/base.py", line 145, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/christian/.pyenv/versions/3.6.7/envs/drf-json-api/lib/python3.6/site-packages/django/core/handlers/base.py", line 143, in _get_response
    response = response.render()
  File "/home/christian/.pyenv/versions/3.6.7/envs/drf-json-api/lib/python3.6/site-packages/django/template/response.py", line 106, in render
    self.content = self.rendered_content
  File "/home/christian/.pyenv/versions/3.6.7/envs/drf-json-api/lib/python3.6/site-packages/rest_framework/response.py", line 72, in rendered_content
    ret = renderer.render(self.data, accepted_media_type, context)
  File "/home/christian/dev/django-rest-framework-json-api/rest_framework_json_api/renderers.py", line 572, in render
    json_api_meta.update(self.extract_root_meta(serializer, serializer_data))
AttributeError: 'str' object has no attribute 'update'

I didn't find any note that meta should not be used as attribute name. If that actually is advised, it would be great to check the models and throw an Error if a model/serializer with a meta property is found.

[sliverc]

According to the specification only name type and id are not allowed but meta should be. See https://jsonapi.org/format/#document-resource-object-fields

This might be a bit tricky to implement though.

[sliverc]

@czosel
Feel free to work on it if you find time.

[sliverc]

As this is an unfinished PR I have created an issue #710 referencing this PR as example.

Closing this PR but contributions are still welcome 😄