If the scorer passed to `_cross_val_score` may be a predefined scorer, or else an instance of:
```python
class _passthrough_scorer(object):
   def __call__(self, estimator, *args, **kwargs):
       return estimator.score(*args, **kwargs)
```
then how would cloning the estimator inhibit `scorer(cloned_estimator, X_test, y_test)` from working?