* request.formats is much simpler now
    * For XHRs or Accept headers with a single item, we use the Accept header
    * For other requests, we use params[:format] or fallback to HTML
    * This is primarily to work around the fact that browsers provide completely
      broken Accept headers, so we have to whitelist the few cases we can
      specifically isolate and treat other requests as coming from the browser
    * For APIs, we can support single-item Accept headers, which disambiguates
      from the browsers
  * Requests to an action that only has an XML template from the browser will
    no longer find the template. This worked previously because most browsers
    provide a catch-all */*, but this was mostly accidental behavior. If you
    want to serve XML, either use the :xml format in links, or explicitly
    specify the XML template: render "template.xml".