Separate redirects and flash messages in navigational_formats and flashing_formats

With

```ruby
  after_filter :add_flash_to_header_if_xhr
  def add_flash_to_header_if_xhr
    if request.xhr? && flash.present?
      response.headers["X-Flash-Messages"] = flash.to_hash.to_json
      flash.discard
    end
  end
```

it is desirable to get flash messages on formats that are not navigational. This patch adds a new option flashing_formats that allows to specify which formats should receive flash messages, by default it inherits the navigational_formats.