Fix child field instantiation with missing arguments

[daluege]

docgenerator.py:

    def _find_field_serializers(self, serializers, found_serializers=set()):
        """
        Returns set of serializers discovered from fields
        """
        ...
            fields = serializer().get_fields()

provokes an instantiation error (AssertionError: Theslug_fieldargument is required.), when a ListSerializer is used with a child field that is not a serializer and that expects mandatory arguments in its constructor.

django-rest-swagger wrongly assumes the child to be a serializer and thus to be callable without arguments.

Example:

class ProfileSerializer(serializers.ModelSerializer):
    interests = serializers.ListSerializer(child=serializers.SlugRelatedField(slug_field='title', queryset=Discipline.objects.all()))

Therefore, add a test for child to inherit from BaseSerializer.