Etags must be quoted according to http spec

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/storage/blobserver.go

@@ -47,7 +47,7 @@ func (bs *blobServer) ServeBlob(ctx context.Context, w http.ResponseWriter, r *h
 		}
 		defer br.Close()
 
-		w.Header().Set("ETag", desc.Digest.String()) // If-None-Match handled by ServeContent
+		w.Header().Set("ETag", fmt.Sprintf(`"%s"`, desc.Digest)) // If-None-Match handled by ServeContent
 		w.Header().Set("Cache-Control", fmt.Sprintf("max-age=%.f", blobCacheControlMaxAge.Seconds()))
 
 		if w.Header().Get("Docker-Content-Digest") == "" {