server.unbind fails to trigger

A short reproducer:

    #!/usr/bin/env node
    var server = require("./lib/index").createServer();
    server.bind('o=foo', function(req, res, next) {
        console.log("bind");
        res.end();
        return next();
    });
    server.search("o=foo", function(req, res, next) {
        console.log("search");
        res.end();
        return next();
    });
    server.unbind(function(req, res, next) {
        console.log("unbind");
        return next();
    });
    server.listen(1389);

When you run this server and run a search with a bind, like so:

    ldapsearch -x -h localhost:1389 -D o=foo -w x -b o=foo

it should've output `bind`, `search` and `unbind`, but it only outputs `bind` and `search`.

I've tracked it down deep into the depths of the server code where the code determining what to do with the unbind request returns "a function that returns a handler" instead of returning the handler. See my commit. I'm fairly new with javascript and not sure if this is good practice or not.