We were testing the URL of the image changes to `missing.png`, but
actually that's confusing because the image record is now invalid and so
its changes can't be saved. That means that, when rendered in the
browser, the image won't render the `missing.png` image but will try to
render the destroyed one.
If we want to render the `missing.png` image when the attachment has
been destroyed, we need to remove the attachment presence validation or
change the `url` method so it detects when an attachment is missing.
04585d289c