A DESCRIPTION OF THE PROBLEM :
This violates RFC7230:
- "A server MUST NOT send a Content-Length header field in any response
   with a status code of 1xx (Informational) or 204 (No Content)."

and com.sun.net.httpserver.HttpExchange#sendResponseHeaders JavaDocs:
- "if {@literal <= -1}, then no response body length is specified and no response body may be written."

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached source code and then:

curl -i "http://localhost:8000/"

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
HTTP/1.1 204 No Content
Date: Sun, 30 Sep 2018 17:08:29 GMT
ACTUAL -
HTTP/1.1 204 No Content
Date: Sun, 30 Sep 2018 17:08:29 GMT
Content-length: 0

---------- BEGIN SOURCE ----------
import com.sun.net.httpserver.HttpExchange;
import com.sun.net.httpserver.HttpHandler;
import com.sun.net.httpserver.HttpServer;

import java.io.IOException;
import java.net.InetSocketAddress;

public class Test {
    public static void main(String[] args) throws Exception {
        HttpServer server = HttpServer.create(new InetSocketAddress(8000), 0);
        server.createContext("/", new MyHandler());
        server.setExecutor(null);
        server.start();
    }

    static class MyHandler implements HttpHandler {
        @Override
        public void handle(HttpExchange t) throws IOException {
            t.sendResponseHeaders(204, -1);
        }
    }
}

---------- END SOURCE ----------

FREQUENCY : always