[Patch] Metals should be loaded from Engines

I'd like to bundle a simple "/backlog" request to my RudeQueue plugin (http://github.com/matthewrudy/ru...

and I'd like to do it as a metal

class RudeQueue::Backlog
  def self.call(env)
    if env["PATH_INFO"] =~ /^\/backlog/
      [200, {"Content-Type" => "text/html"}, [RudeQueue.backlog.to_s]]
    else
      [404, {"Content-Type" => "text/html"}, ["Not Found"]]
    end
  end
end

it seems to make sense,

however Engines don't currently load their Metal.

Here's my fork which fixes that http://github.com/matthewrudy/ru...

and a patch is also attached.

Any suggestions on how to better format the code, or manage the paths is very much welcome. (notably it seems to be very hard to test in code... I've tested it manually)