User-agent ignored in WebRequest.defaults

```
WebRequest.defaults({ headers: {'User-Agent': 'curl/7.28.1'} });
var data = await WebRequest.get('http://localhost');
```

The request made over the wire:
```
GET / HTTP/1.1
host: localhost
Connection: close
```

What am i doing wrong? `User-Agent` doesn't make it in..

Using `web-request 1.0.7` from npm.