addressing the review

client.go

@@ -190,8 +190,8 @@ func (d *Dialer) DialContext(ctx context.Context, urlStr string, requestHeader h
 		for _, proto := range d.TLSClientConfig.NextProtos {
 			if proto != "http/1.1" {
 				return nil, nil, fmt.Errorf(
-					`protocol %q was given but is not supported;
-					sharing tls.Config with net/http Transport can cause this error`,
+					"websocket: protocol %q was given but is not supported; "+
+						"sharing tls.Config with net/http Transport can cause this error",
 					proto,
 				)
 			}

client_server_test.go

@@ -1098,3 +1098,39 @@ func TestNetDialConnect(t *testing.T) {
 		}
 	}
 }
+
+func TestNextProtos(t *testing.T) {
+	ts := httptest.NewUnstartedServer(
+		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {}),
+	)
+	ts.EnableHTTP2 = true
+	ts.StartTLS()
+	defer ts.Close()
+
+	d := Dialer{
+		TLSClientConfig: ts.Client().Transport.(*http.Transport).TLSClientConfig,
+	}
+
+	r, err := ts.Client().Get(ts.URL)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	r.Body.Close()
+
+	// Asserts that Dialer.TLSClientConfig.NextProtos contains "h2"
+	// after the Client.Get call from net/http above.
+	var containsHTTP2 bool = false
+	for _, proto := range d.TLSClientConfig.NextProtos {
+		if proto == "h2" {
+			containsHTTP2 = true
+		}
+	}
+	if !containsHTTP2 {
+		t.Fatalf("Dialer.TLSClientConfig.NextProtos does not contain \"h2\"")
+	}
+
+	_, _, err = d.Dial(makeWsProto(ts.URL), nil)
+	if err == nil {
+		t.Fatalf("Dial succeeded, expect fail ")
+	}
+}