Rails Metal Pass Through Response Code Should be Configurable

I've created a patch for Rails Metal so that the Response Code that is checked to see if the response should continue through the rest of the stack is configurable. You can set it to an array of HTTP Response codes, or just a single one, much like what can be done with Rack::Cascade (http://github.com/rack/rack/blob/fd1c1d2e494d9a7d15e0458602ef121f24...) [credit to Josh Peek for pointing me towards Rack::Cascade]

Anyways, this lets you configure something like 417 to be the pass through response code so your metal can return 404s if you want them to.