Raise validation when the field is not mapped

[davinogueiradev]

What's a better way to validate fields in a restricted mode like this:

class DummySerializer(serializers.Serializer):
    foo = serializers.CharField()

DummySerializer(data={"foo": "bar"}).is_valid(raise_exception=True) # is valid, not raise exception
DummySerializer(data={"foo": "bar", "bar": "foo"}).is_valid(raise_exception=True) # raise exception, because bar is not in fields