@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);
                }
User-agent header should be overridden by ClientOption.HTTP_HEADER
core/src/test/java/com/linecorp/armeria/client/HttpClientIntegrationTest.java