Response Caching should ignore the host part of the request

Ideally, response caching should really only take notice from the path onwards (with all the great options for query parameters and headers of course) sine `http://mydomain.com/images/logo.png` and `https://mydomain.com/images/logo.png` and `http://192.168.1.1/images/logo.png` and `https://www.mydomain.com/images/logo.png` should be the same resource (since they are actually the same file) and only cache once (and when one is replaced, so should all the others).

So I thought maybe I could just register a new `IResponseCachingKeyProvider` that provided a different implementation of the key, but all the interfaces are `internal` - so much of open for extensibility :-(