koa的option请求404问题

post请求以前会先发出一个options请求，若是服务端没有返回200，就会报404错误，配置方法以下

app.use(async (ctx, next)=> {
    ctx.set('Access-Control-Allow-Origin', '*');
    ctx.set('Access-Control-Allow-Headers', 'Content-Type, Content-Length, Authorization, Accept, X-Requested-With');
    ctx.set('Access-Control-Allow-Methods', 'PUT, POST, GET, DELETE, OPTIONS');
    if (ctx.method == 'OPTIONS') {
      ctx.body = 200; 
    } else {
      await next();
    }
});
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