// cache-then-refresh
if (req.headers.get('x-refresh')) {
  const results$ = sendRequest(req, next, this.cache);
  return cachedResponse ?
    results$.pipe( startWith(cachedResponse) ) :
    results$;
}
// cache-or-fetch
return cachedResponse ?
  of(cachedResponse) : sendRequest(req, next, this.cache);