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`