Handler for Range requests. This is a generic handler that can handle range requests to any resource
of a fixed content length i.e. any resource where the content-length header has been set.
Note that this is not necessarily the most efficient way to handle range requests, as the full content
will be generated and then discarded.
At present this handler can only handle simple (i.e. single range) requests. If multiple ranges are requested the
Range header will be ignored.