Well my original use of setting a value to null was to delete it, without using delete on the object key.

But yes you are right. Maybe it should do the exact same thing as jQuery, but I don't think it should return a serialized 'null' value.

So this test could be rewritten to be:
```
expect(toKeyValue({nullKey: null, key: 'value'})).toEqual('nullKey=&key=value')
```

I would be happy to make this PR use the jQuery method as it seems that it is the excepted format. 

Yes this can be accomplished today with just setting nullKey = ''; which is a solution that requires no changes.

Also maybe we should also consider the 'undefined' condition as well.