Correctly handle the update_fields kwarg on Category.save()

If the update_fields is given as kwarg and doesn’t contain the slug as
field to update then skip validating uniqueness of the slug and updating
the children categories.

I also think that we can deprecate the update_slugs kwarg from the save method and move it within the added condition. 
Well if update_fields is not specified then all fields are updated. So we have to check if it is specified, and if it is then if 'slug' is in list.

Good regarding de `name` field. We normally decouple name and slug in projects so I didn't think about it :-)