Bug report

Describe the bug

You get a 500 with code in /pages/api/index.js but not if you put the same code in /pages/api/foo.js

To Reproduce

Put the following code in /pages/api/index.js:

export default (req, res) => {
  res.setHeader('Content-Type', 'application/json');
  res.statusCode = 200;
  res.end(JSON.stringify({ name: 'Nextjs' }));
};

Deploy to Now, and go to deployment.endpoint/api, you'll get a 500 (complaining that res.setHeader is not a function).

Hope I'm not missing something. Cheers!