Return a json response when a incomplete url is requested

[philip-hunt]

#406

[philip-hunt]

The existing code determined if the url was 'incomplete' by checking to see if the request had a file extension. If not it assumed it was a directory. I've kept this existing functionality, but is this what we want? It would mean that the CDN would never return any files without an extension.

[jimlambie]

@philip-hunt you raise an interesting question... we recently disabled the "jpg by default" feature so that we can effectively use CDN as a passthrough HTML cache. For example, configuring CDN to obtain assets from a remote source such as https://dadi.cloud/en/ would allow you to serve the entire website through CDN, caching HTML responses on the way and therefore serving the full site from the CDN cache.

This approach relies on being able to specify paths with no extension - however it's probably the case that there are no tests in the project for this functionality.

Let me add tests for this first, and we'll come back to this.

cc @eduardoboucas