if query params are empty, then request path shouldn't end with a '?' (merges cleanly now)

If you do this:

```javascript
req = request.get({ uri: 'http://www.google.com', qs: {}})`
```

Then req.path equates to /?, which only makes sense if there are some query parameters.

Opening a new PR since issue https://github.com/mikeal/request/pull/436 wasn't merging cleanly.