@BeforeAll
        void startJetty() throws Exception {
            jetty = new org.eclipse.jetty.server.Server(0);
            jetty.setHandler(new AbstractHandler() {
                @Override
                public void handle(String target, Request baseRequest, HttpServletRequest request,
                                   HttpServletResponse response) throws IOException, ServletException {
                    if (Collections.list(request.getHeaders("host")).size() == 1) {
                        response.setStatus(HttpStatus.OK.code());
                    } else {
                        response.sendError(HttpStatus.INTERNAL_SERVER_ERROR.code());
                    }
                    baseRequest.setHandled(true);
                }
            });
            jetty.start();
        }
User-agent header should be overridden by ClientOption.HTTP_HEADER
core/src/test/java/com/linecorp/armeria/client/HttpClientIntegrationTest.java