Cleanup `ActionController::Rendering`
Update guides for new rendering options

* Introduces `:plain`, `:html`, `:body` render option.
* Update guide to use `render :plain` instead of `render :text`.
Add missing CHANGELOG entry to Action View
Add `#no_content_type` attribute to `AD::Response`

Setting this attribute to `true` will remove the content type header
from the request. This is use in `render :body` feature.