Introduces "optional" requests. These should be fulfilled by the FileSource only there's a low-cost/easy way to obtain the data (e.g. from a local cache). If the data for an optional request cannot be found, it *must* return a Response object with a NotFound error.
Traditional "required" requests still work the same way, with one change: If you set any prior* field in the Resource (i.e. priorModified, priorEtag, or priorExpires), the DefaultFileSource assumes that you already have the cache value and will not consult the cache before performing the request. If a prior cache lookup didn't turn up any data, and you therefore don't have an Etag or Modified value, you can still skip the cache by setting priorExpires. This will of course always result in a non-conditional HTTP request.
