Introduce `render :body` to render raw content

Introduce `render :body` as an option to send 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. Please see RFC 2616, section 7.2.1 for more
detail.

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

Introduce `render :text` as an option to 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.