unescaped characters'

It appears most browsers (tested in Firefox, Safari and
Chromium[Chrome/Brave]) will url-encode paths automatically if they are
not already encoded.

`xhr2` doesn't match that behavior when run in node: using an
unencoded URL will result in `TypeError [ERR_UNESCAPED_CHARACTERS]`.

Just adding the encoding line however causes a new issue; already
encoded URLs end up being double-encoded, changing the URL incorrectly.

Using [`native-url`](https://github.com/GoogleChromeLabs/native-url
instead of `url` fixes this, as well as removing the dependency on the
deprecated [Legacy URL
API](https://nodejs.org/api/url.html#url_legacy_url_api).

It would have of course been better to refactor `xhr2` to not use the
deprecated API, however this does not appear to be trivial looking at
the [`native-url`
code](https://github.com/GoogleChromeLabs/native-url/tree/master/src).