godoc: fix redirect loop for URL "/".

R=golang-dev, bradfitz, rsc, adg
CC=golang-dev
https://golang.org/cl/5606045

src/cmd/godoc/godoc.go

@@ -743,7 +743,11 @@ func applyTemplate(t *template.Template, name string, data interface{}) []byte {
 }
 
 func redirect(w http.ResponseWriter, r *http.Request) (redirected bool) {
-	if canonical := path.Clean(r.URL.Path) + "/"; r.URL.Path != canonical {
+	canonical := path.Clean(r.URL.Path)
+	if !strings.HasSuffix("/", canonical) {
+		canonical += "/"
+	}
+	if r.URL.Path != canonical {
 		http.Redirect(w, r, canonical, http.StatusMovedPermanently)
 		redirected = true
 	}