c14n fix/imported raw failed URLs fix

This only came to light because of an unrelated issue with `Raw::FailedUrl` not belonging to its `Raw::ImportedFile` correctly, for which a failing spec was written which highlighted an `optic14n` issue where some paths were being discarded.

`rake raw:wipe raw:import_urls`

should be run after this hits preview

Bugs fixed:

https://www.pivotaltracker.com/s/projects/860575/stories/54502932
https://www.pivotaltracker.com/s/projects/860575/stories/54509212