@pothibo `request.format` is the *requested* format, not the response Content-Type.

For example, in `#test_should_only_allow_same_origin_js_get_with_xhr_header`:
```ruby
> [content_type, request.format, rendered_format]
=> ["text/javascript", #<Mime::Type:0x007ff4d96d15e8 @synonyms=["application/xhtml+xml"], @symbol=:html, @string="text/html">, #<Mime::NullType:0x007ff4d8438530>]
```