add etag only for PUT or large chunked uploads

weed/server/volume_server_handlers_write.go

@@ -166,3 +166,11 @@ func setEtag(w http.ResponseWriter, etag string) {
 		}
 	}
 }
+
+func getEtag(resp *http.Response) (etag string){
+	etag = resp.Header.Get("ETag")
+	if strings.HasPrefix(etag, "\"") && strings.HasSuffix(etag, "\""){
+		return etag[1:len(etag)-1]
+	}
+	return
+}