dont gzip a gzip and .js.gz has contenttype javascript

gzipresponsewriter.go

@@ -12,6 +12,9 @@ type GZipResponseWriter struct {
 }
 
 func Can(r *http.Request) bool {
+	if strings.HasSuffix(r.URL.Path, ".gz") {
+		return false
+	}
 	encodings, _ := r.Header["Accept-Encoding"]
 	for _, encoding := range encodings {
 		items := strings.Split(encoding, ",")