Fix a bug with b.require if {expose:true}

If I expose a file without giving a name ({expose:true}), hoping for it to use the file hash, a bug may happen inside the bundle : the file is exposed with hash, but internally referenced with id number.

At https://github.com/substack/node-browserify/blob/e7fc5cb470a3bef22328f69e5c5461549dbede36/index.js#L217
If the exposed file was already seen in a dependency, an id number was generated and the hash is not used here.