cmd/golangorg: add missing /dl/ redirect on tip.golang.org

Due to a missing redirect there is unfortunately
a 404 when trying to download golang from
the subdomain tip.golang.org.

Change-Id: Ia14f419152769ccdf1ef5ede29a0d0e11c6bae6a
GitHub-Last-Rev: a825e2183a6bf9556de235b589af23a35d025153
GitHub-Pull-Request: golang/website#85
Reviewed-on: https://go-review.googlesource.com/c/website/+/345591
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jamal Carvalho <jamal@golang.org>
Website-Publish: DO NOT USE <dmitshur@google.com>
diff --git a/cmd/golangorg/server.go b/cmd/golangorg/server.go
index 885e549..1f29956 100644
--- a/cmd/golangorg/server.go
+++ b/cmd/golangorg/server.go
@@ -206,6 +206,7 @@
 	// Register a redirect handler for /dl/ to the golang.org download page.
 	// (golang.org/dl and golang.google.cn/dl are registered separately.)
 	mux.Handle("/dl/", http.RedirectHandler("https://golang.org/dl/", http.StatusFound))
+	mux.Handle("tip.golang.org/dl/", http.RedirectHandler("https://golang.org/dl/", http.StatusFound))
 
 	godev, err := godevHandler(godevFS)
 	if err != nil {
diff --git a/cmd/golangorg/testdata/web.txt b/cmd/golangorg/testdata/web.txt
index b80008e..70a64fe 100644
--- a/cmd/golangorg/testdata/web.txt
+++ b/cmd/golangorg/testdata/web.txt
@@ -103,6 +103,9 @@
 GET https://tip.golang.org/pkg/
 redirect == https://pkg.go.dev/std@master
 
+GET https://tip.golang.org/dl/
+redirect == https://golang.org/dl/
+
 GET https://golang.org/pkg?m=old
 redirect == /pkg/?m=old