Null fields not included in FormData, unlike jQuery

I'm making POST requests like this:

```
request.post(uri)
.set('Content-Type', 'application/x-www-form-urlencoded')
.send({myFlag: 'dunce'})
.end((err, result) => { ....
```

That works fine, and the FormData includes `myFlag` as you'd expect. But if I instead send `{myFlag: null}` then the FormData contains no reference at all to `myFlag`. Previously I was using `jQuery.ajax`, and in that case the FormData would still contain a reference to `myFlag`, but obviously with an empty value. SuperAgent breaks my server calls, which insist on seeing `myFlag`.

Since this behavior is inconsistent with jQuery, I'm wondering whether there's an option to align them, or is one of jQuery or SuperAgent doing this incorrectly?