Missing Content-Type header

The initial response for the html page is missing `Content-Type: text/html; charset=utf-8`.

I think the reason this is happening is `express-http-server.js` has the following code:

```
52 if (this.distPath) {
53 app.get('/', fastbootMiddleware);
54 app.use(express.static(this.distPath));
55 app.get('/assets/*', function(req, res) {
56 res.sendStatus(404);
57 });
58 }
```

`app.get('/', fastbootMiddleware);` causes the initial request to not have the correct content-type since that is normally handled by `express.static`.

So it seems [fastboot-express-middleware](https://github.com/ember-fastboot/fastboot-express-middleware) needs to call `.type('text/html')` somewhere.