The recent HTTP client refactor in 14e25ab9fe
caused feed refreshes to no longer make conditional requests. Prior to
the refactor, `client.WithCacheHeaders` handled this. Now this function
is split into `fetcher.WithETag` and `fetcher.WithLastModified` but
these functions are only declared and never actually used. Fix this by
calling them inside `handler.RefreshFeed`.
fe0ef8b579