Rook Reference Classes break on `onHeaders`

Hi,

I've written a fix for a situation I ran into where a reference class-based app would break.

What was happening was there was a check for `NULL`, and because there is no field or method present in the Reference Class, an error is returned.

A simple app that breaks follows:

```r
Builder$new(
    Static$new(urls = c('/js', '/css'),
               root = '.'),
    Brewery$new(url = '/brew',
               root = '.'),
    Redirect$new("/brew/index.html"))
```

I'm not sure if my fix is suitable but I figure it's better than nothing :).