`render nothing: true` or rendering a `nil` body no longer add a single
space to the response body.
The old behavior was added as a workaround for a bug in an early version of
Safari, where the HTTP headers are not returned correctly if the response
body has a 0-length. This is been fixed since and the workaround is no
longer necessary.
Use `render body: ' '` if the old behavior is desired.
This is an option for to 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.