Q.ninvoke() returns undefined

I am promisifying a node-style function using Q.ninvoke(), but it returns undefined. This is a surprise to me:

``` javascript
var promise = Q.ninvoke(NginxConfFile, 'create', NGINX_UPSTREAM_SERVICES_FILE)
    .then(function(conf) {
    ....
```

promise is undefined (but the ninvoke function calls the .then() function correctly).

I'll just create use a deferred object using Q.defer() for now.
