TypeError: res.writeHead is not a function

Hi, I've done this with your latest code:

> npm run build
> npm run start

Right after the _Open http://localhost:8080 in a browser to view the app._ message, I get an exception:

```
[0] /Users/me/projects/react-redux-universal-hot-example/src/server.js:105
[0] res.writeHead(500, { 'content-type': 'application/json' });
[0] ^
[0]
[0] TypeError: res.writeHead is not a function
[0] at ProxyServer. (/Users/me/projects/react-redux-universal-hot-example/src/server.js:55:9)
```

This is the code that fails:

```
// added the error handling to avoid https://github.com/nodejitsu/node-http-proxy/issues/527
proxy.on('error', (error, req, res) => {
let json;
if (error.code !== 'ECONNRESET') {
console.error('proxy error', error);
}
if (!res.headersSent) {
res.writeHead(500, {'content-type': 'application/json'}); // <- EXCEPTION
}
```

I installed the latest http-proxy package but nothing changed.

Thanks,
Alvaro