message: set assert! message when misusing cache control
diff: @@ -101,9 +101,10 @@ impl AfterMiddleware for CacheMiddleware {
  .unwrap_or(&CachePolicy::NoCaching);
  if cfg!(test) {
  - // handlers should never set their own caching headers and
  - // only use the caching header templates above.
  - assert!(!res.headers.has::<CacheControl>());
  + assert!(
  + !res.headers.has::<CacheControl>(),
  + "handlers should never set their own caching headers and only use CachePolicy to control caching."
  + );
  }
  res.headers.set(CacheControl(cache.render(config)));
  