Fixes a bug making userOptions to be ignored

@yannickcr, really impressive how promptly you have developed the API wrapper, given that the API spec was released yesterday

Trying to play with the library, but when passing a user option ```{dir: folderPath}```, it is ignored by the library (maybe, I'm doing it wrong?):

```javascript
        app.btsync.addFolder(
          {dir: folderPath},
          function(err, data) {
            if (err) app.logger.error(err);
            app.logger.info(data);
        });
```

Current change fixes the issue. @yannickcr could you please review and merge?