req.fresh throws when no response headers are set

Currently any request to a simplistic Express app like so will fail with a 500 because [jshttp/fresh](https://github.com/jshttp/fresh) does not expect a response's headers to be `undefined`:

```javascript
(function () {
  'use strict';

  var app = require('express')();

  app.disable('x-powered-by'); // no default headers are set, so `res.headers` is now `undefined`

  app.get('/', function (req, res) {
    res.status(204).send();
  });

  module.exports = app;

}());
```

This Pull Request ensures that at the very least an empty hash is sent to [jshttp/fresh](https://github.com/jshttp/fresh) to prevent requests to that application from erroring out with a 500.