net/http/cgi: set SERVER_PORT to 443 when req.TLS != nil

A hostname without a port leaves the port implied by the protocol.
For HTTPS, the implied port is 443, not 80.

Change-Id: I873a076068f84c8041abf10a435d9499635730a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/454975
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/http/cgi/host.go

@@ -132,6 +132,9 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 
 	port := "80"
+	if req.TLS != nil {
+		port = "443"
+	}
 	if matches := trailingPort.FindStringSubmatch(req.Host); len(matches) != 0 {
 		port = matches[1]
 	}