Add handle_request property for Express 4.6 compatibility

As of Express 4.6, the Layer object that gets inserted into the front
of the app's middleware stack in `passportStub.install()` needs a callable
handle_request method. Running passportStub.install() in Express 4.6 or later will throw
the error `Uncaught TypeError: Object #<Object> has no method 'handle_request'`
See https://github.com/visionmedia/express/blob/4.6.0/lib/router/layer.js, line 67