stringify with nested objects and null values does not work correctly

qs generates incorrect querystring when nested object contains null or undefined values :

```javascript
obj = {foo: 1, nested: { bar: null, qux: true } }
qs.stringify(obj) // => foo=1&bar=&nested[qux]=true
``` 

See that bar parameter is not nested :( 

I've attached failing test for this scenario.