[wleeaf]

When the server rejects a WebSocket handshake, WebSocketHandshake::sendErrorResponse writes the status line and the Server header, but not the empty line that ends the header section. For example, a request without Sec-WebSocket-Key gets exactly these bytes before the connection closes:

HTTP/1.1 400 Missing Sec-WebSocket-Key value\r\n
Server: ixwebsocket/12.1.0 linux nossl zlib 1.3\r\n

That's an incomplete HTTP response. A client can only treat it as finished once the connection drops, and strict HTTP clients report it as truncated.

Also, a client asking for a WebSocket version other than 13 gets a 400 without a Sec-WebSocket-Version header. RFC 6455 section 4.2.2 says that in this case the server sends an appropriate error code (such as 426 Upgrade Required) and a Sec-WebSocket-Version header with the versions it understands, so the client can retry with one of them (section 4.4).

This change:

• makes sendErrorResponse end the header section, and adds an optional extraHeaders argument;
• answers an unsupported Sec-WebSocket-Version with 426 and Sec-WebSocket-Version: 13. The reason text and the WebSocketInitResult error message are unchanged.

The new section in test/IXWebSocketServerTest.cpp sends Sec-WebSocket-Version: 8. It expects a 426, the Sec-WebSocket-Version: 13 header, and the empty line that ends the headers. Without the change it fails (400 == 426). All 22 ctest tests pass with the change (Linux, USE_TLS=0), and git clang-format reports nothing for the change.