Non-kwargs requests are deprecated now.
Guides are updated as well.
`post url, nil, nil, { a: 'b' }` doesn't make sense.
`post url, params: { y: x }, session: { a: 'b' }` would be an explicit way to do the same
* 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".
