@Override
                public void onNext(HttpObject o) {
                    if (notified) {
                        // Discard the first response.
                        return;
                    }

                    notified = true;
                    assert upgradeEvt == UpgradeEvent.UPGRADE_SUCCESSFUL;
                    onUpgradeResponse(ctx, true);
                }
Sends the initial upgrade request, which is {@code "OPTIONS * HTTP/1.1"}.
core/src/main/java/com/linecorp/armeria/client/HttpClientPipelineConfigurator.java