Respond to preflight requests with CORS

src/server.ts

@@ -55,7 +55,7 @@ const server = http.createServer((req, res) => {
       clients[jobId] = res;
       break;
     default:
-      res.writeHead(404);
+      res.writeHead(404, CORS);
       res.end('404!');
   }
 });