  if err != nil {   t.Fatal(err)  }  cache := newCacheRoundTripper(cacheDir, rt)   // First call, caches the response  req := &http.Request{   Method: http.MethodGet,   URL:    &url.URL{Host: "localhost"},  }  rt.Response = &http.Response{   Header:     http.Header{"ETag": []string{`"000000"`}},   Body:       ioutil.NopCloser(bytes.NewReader([]byte("Content"))),   StatusCode: http.StatusOK,  }  resp, err := cache.RoundTrip(req)  if err != nil {   t.Fatal(err)  }  content, err := ioutil.ReadAll(resp.Body)  if err != nil {   t.Fatal(err)  }  if string(content) != "Content" {   t.Errorf(`Expected Body to be "Content", got %q`, string(content))  }   