Introduce `render :body` for render raw content

This is an option for sending a raw content back to browser. Note that
this rendering option will unset the default content type and does not
include "Content-Type" header back in the response.

You should only use this option if you are expecting the "Content-Type"
header to not be set. More information on "Content-Type" header can be
found on RFC 2616, section 7.2.1.

Please see #12374 for more detail.
Update hash format for render_text_test
Introduce `render :plain` for render plain text

This is as an option for render content with a content type of
`text/plain`. This is a preferred option if you are planning to render a
plain text content.

Please see #12374 for more detail.
Introduce `render :html` for render HTML string

This is an option for render HTML content with a content type of
`text/html`. This rendering option calls `ERB::Util.html_escape`
internally to escape unsafe HTML string, so you will have to mark your
string as html safe if you have any HTML tag in it.

Please see #12374 for more detail.
Fix a fragile test on `action_view/render`

This test were assuming that the list of render options will always be
the same. Fixing that so this doesn't break when we add/remove render
option in the future.
Cleanup `ActionController::Rendering`