fix for express not exposing .close()

Right now supertest throws an error when it tries to close an express server. Express doesn't expose `.close()`, but it does return the underlying http server on `.listen()`: http://stackoverflow.com/questions/14515954/how-to-properly-close-node-express-server/14516195#14516195

This will save a reference to that underlying http server and use it if `close` isn't available.