`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.