Add keyword arguments to CT get_for_model

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/acls/serializers.py

@@ -171,7 +171,7 @@ class WritableAccessControlListSerializer(serializers.ModelSerializer):
 
     def validate(self, attrs):
         attrs['content_type'] = ContentType.objects.get_for_model(
-            self.context['content_object']
+            model=self.context['content_object']
         )
         attrs['object_id'] = self.context['content_object'].pk