@jbaudanza While testing this in development, I got this `Rack::Lint` error. Apparently, the `Content-Type` shouldn't be set for 304 Not Modified response.

![image](https://f.cloud.github.com/assets/181187/895268/f3b19a7c-fac8-11e2-9228-890667f1c917.png)

I solved this by changing this line to:

```ruby
elsif status != 304
```

I don't think this is the best solution. I'd be happy to propose a more robust solution if the maintainers of Rack have any interest in this pull-request.