http2: also set "http/1.1" ALPN in ConfigureServer

With CL 289209, we started enforcing ALPN protocol overlap when both
sides support ALPN. This means that an "http/1.1"-only ALPN-aware client
will fail to connect to a server with only "h2" in NextProtos.
Unfortunately, that's how ConfigureServer was setting up the tls.Config.

The new behavior mirrors ConfigureTransport on the client side (but not
Transport.newTLSConfig because the latter is used to make HTTP/2-only
connections).

Updates golang/go#46310

Change-Id: Idbab7a6f873f3be78104df6f2908895f4d399bd3
Reviewed-on: https://go-review.googlesource.com/c/net/+/325611
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

Author: FiloSottile

http2/server.go

@@ -259,16 +259,12 @@ func ConfigureServer(s *http.Server, conf *Server) error {
 
 	s.TLSConfig.PreferServerCipherSuites = true
 
-	haveNPN := false
-	for _, p := range s.TLSConfig.NextProtos {
-		if p == NextProtoTLS {
-			haveNPN = true
-			break
-		}
-	}
-	if !haveNPN {
+	if !strSliceContains(s.TLSConfig.NextProtos, NextProtoTLS) {
 		s.TLSConfig.NextProtos = append(s.TLSConfig.NextProtos, NextProtoTLS)
 	}
+	if !strSliceContains(s.TLSConfig.NextProtos, "http/1.1") {
+		s.TLSConfig.NextProtos = append(s.TLSConfig.NextProtos, "http/1.1")
+	}
 
 	if s.TLSNextProto == nil {
 		s.TLSNextProto = map[string]func(*http.Server, *tls.Conn, http.Handler){}