Calls a `get_root_meta` function on a serializer, if it exists.

def extract_root_meta(cls, serializer, resource):
        """
        Calls a `get_root_meta` function on a serializer, if it exists.
        """
        many = False
        if hasattr(serializer, 'child'):
            many = True
            serializer = serializer.child

        data = {}
        if getattr(serializer, 'get_root_meta', None):
            json_api_meta = serializer.get_root_meta(resource, many)
            assert isinstance(json_api_meta, dict), 'get_root_meta must return a dict'
            data.update(json_api_meta)
        return data