Files/Items should report to respond_to? methods that they respond to

Otherwise this can lead to confusing behavior:

```
> item.id
> "12345"
> item.try(:id)
> nil
> item.respond_to?(:id)
> false
```

See https://robots.thoughtbot.com/always-define-respond-to-missing-when-overriding

Corrective PR: #118