Address review comments

Author: lukka5

django_declarative_apis/machinery/filtering.py

@@ -92,7 +92,7 @@ def _cache_related_instance(inst, field_name, model_cache):
     else:
         logger.debug("ev=model_cache, status=miss, key=%s", cache_key)
         related_instance = getattr(inst, field_name)
-        if not isinstance(related_instance, models.Model):
+        if not isinstance(related_instance, val_cls):
             return
         model_cache[cache_key] = related_instance
     setattr(inst, field_name, related_instance)