Ruby: added support for rack V3.

Ruby applications would fail to start if they were using rack v3

  2022/09/28 15:48:46 [alert] 0#80912 [unit] Ruby: Failed to parse rack script
  2022/09/28 15:48:46 [notice] 80911#80911 app process 80912 exited with code 1

This was due to a change in the rack API

Rack V2

  def self.load_file(path, opts = Server::Options.new)
    ...
    cfgfile.sub!(/^__END__\n.*\Z/m, '')
    app = new_from_string cfgfile, path

    return app, options
  end

Rack V3

  def self.load_file(path)
    ...

    return new_from_string(config, path)
  end

This patch handles _both_ the above APIs by correctly handling the cases
where we do and don't get an array returned from
nxt_ruby_rack_parse_script().

Closes: <https://github.com/nginx/unit/issues/755>
Tested-by: Andrew Clayton <a.clayton@nginx.com>
Reviewed-by: Andrew Clayton <a.clayton@nginx.com>
[ Andrew: Patch by Zhidao, commit message by me with input from Zhidao ]
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

docs/changes.xml

@@ -31,6 +31,12 @@ NGINX Unit updated to 1.29.0.
          date="" time=""
          packager="Nginx Packaging <nginx-packaging@f5.com>">
 
+<change type="feature">
+<para>
+support rack v3 in ruby applications.
+</para>
+</change>
+
 <change type="bugfix">
 <para>
 fix HTTP cookie parsing when the value contains an equals sign.