http3: set tls.Config.ServerName for outgoing requests, if unset (#3867)

* fix #3865

* add test case

* Update http3/client, client_test.go

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

---------

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

http3/client.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"strconv"
 	"sync"
@@ -92,6 +93,14 @@ func newClient(hostname string, tlsConf *tls.Config, opts *roundTripperOpts, con
 	} else {
 		tlsConf = tlsConf.Clone()
 	}
+	if tlsConf.ServerName == "" {
+		sni, _, err := net.SplitHostPort(hostname)
+		if err != nil {
+			// It's ok if net.SplitHostPort returns an error - it could be a hostname/IP address without a port.
+			sni = hostname
+		}
+		tlsConf.ServerName = sni
+	}
 	// Replace existing ALPNs by H3
 	tlsConf.NextProtos = []string{versionToALPN(conf.Versions[0])}