net/http: add StripPrefix example; simplify code

The example is the same as the FileServer one, but
it's relevant for both.

Also use strings.TrimPrefix while I'm here.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7598046

src/pkg/net/http/example_test.go

@@ -54,3 +54,8 @@ func ExampleFileServer() {
 	// we use StripPrefix so that /tmpfiles/somefile will access /tmp/somefile
 	http.Handle("/tmpfiles/", http.StripPrefix("/tmpfiles/", http.FileServer(http.Dir("/tmp"))))
 }
+
+func ExampleStripPrefix() {
+	// we use StripPrefix so that /tmpfiles/somefile will access /tmp/somefile
+	http.Handle("/tmpfiles/", http.StripPrefix("/tmpfiles/", http.FileServer(http.Dir("/tmp"))))
+}

src/pkg/net/http/server.go

@@ -948,13 +948,16 @@ func NotFoundHandler() Handler { return HandlerFunc(NotFound) }
 // request for a path that doesn't begin with prefix by
 // replying with an HTTP 404 not found error.
 func StripPrefix(prefix string, h Handler) Handler {
+	if prefix == "" {
+		return h
+	}
 	return HandlerFunc(func(w ResponseWriter, r *Request) {
-		if !strings.HasPrefix(r.URL.Path, prefix) {
+		if p := strings.TrimPrefix(r.URL.Path, prefix); len(p) < len(r.URL.Path) {
+			r.URL.Path = p
+			h.ServeHTTP(w, r)
+		} else {
 			NotFound(w, r)
-			return
 		}
-		r.URL.Path = r.URL.Path[len(prefix):]
-		h.ServeHTTP(w, r)
 	})
 }