[url] application/x-www-form-urlencoded serializer does not handle enough (#83)

https://html.spec.whatwg.org/#application/x-www-form-urlencoded-encoding-algorithm has a couple of special cases we should just handle here, since we also handle them in the parser for this format. Once that is done we can remove the algorithm from HTML.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/url/issues/83