I solved my problem by adding this function and calling it instead of connectWebSocket call.
/**
Returns a WebSocket client object that is connected to the specified host.
*/
WebSocket simpleConnectWebSocket(URL url, const(HTTPClientSettings) settings = defaultSettings)
@safe {
bool use_tls = (url.schema == "wss") ? true : false;
url.schema = use_tls ? "https" : "http";
/*scope*/auto rng = secureRNG();
auto challengeKey = generateChallengeKey(rng);
auto answerKey = computeAcceptKey(challengeKey);
HTTPClientResponse res = requestHTTP(url,
(scope req) {
req.method = HTTPMethod.GET;
req.headers["Upgrade"] = "websocket";
req.headers["Connection"] = "Upgrade";
req.headers["Sec-WebSocket-Version"] = "13";
req.headers["Sec-WebSocket-Key"] = challengeKey;
},
);
auto key = "sec-websocket-accept" in res.headers;
enforce(key !is null, "Response is missing the Sec-WebSocket-Accept header.");
enforce(*key == answerKey, "Response has wrong accept key");
auto conn = res.switchProtocol("websocket");
auto ws = new WebSocket(conn, null, rng);
return ws;
}