Make sure Rails will always raise `ActionController::RoutingError` when show_exceptions is set to false

Refer to this StackOverflow question: http://stackoverflow.com/questions/5022454/raising-route-not-found-...

I just found that there's a bug in Rails since April 2010 that if you set the config.action_dispatch.show_exceptions to false you wouldn't get ActionController::RoutingError because the middleware that supposed to raise the exception, ActionDispatch::ShowExceptions doesn't get loaded if the config.action_dispatch.show_exceptions is set to false.

This patch will make sure that the middleware will always get loaded. So if the middleware down the stack returns a "X-Cascade: pass" header, it will raise the ActionController::RoutingError as expected.

I've separate the patches into three commits:

1. Make sure that we set env["action_dispatch.show_exceptions"]
   
2. Add failing test case for application not raising ActionController::RoutingError even the route is not found.
   
3. Always use ActionDispatch::ShowExceptions middleware

I would love to see this got committed in both master and 3-0-stable as there's already someone on 3.0.x came across this bug while doing test driven development.

Thank you.