gateway timeout while setting ctx.status=100
I met a problem today

client send a request with header Expect='100-continue'
the server program can't handle it.
then i wrote a middleware to fix it but i doesn't work

```
// my test code
const koa = require('koa');
const app = new koa();

app.use(ctx => {
  ctx.status = 100;
  ctx.body = 'Hello Koa';
});

app.listen(3000);
```

is that a bug?