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. 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.