Code example: 

void foo(HTTPServerRequest req, HTTPServerResponse res)
{
	string str = `[1,2,3]`;
	
	res.statusCode = HTTPStatus.OK;
	res.headers["Content-Type"] = "application/json; charset=UTF-8";
	
	res.bodyWriter.write(str);
	res.bodyWriter.flush();
}

Do I need to call res.bodyWriter.flush explicitly?