Editorial: Add proper serialization of URL properties

[rubycon]

This PR fix missing serialization steps of internal port and host properties in the spec. The URL parser return port as an integer and host as either a hostname string, an integer (IPv4 address) or an array of integers (IPv6 address). For further processing by the URLPattern code these properties need to be properly converted to string.

The issue was discussed here: #202 (comment)

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[jeremyroman]

Thanks for the pull request. Only a couple things before I can merge it.

1. Please amend the commit message according to meta/COMMITTING.md. In particular, the first line should probably be marked as "Editorial" (which lets us avoid many of the usual checks) and be written in the imperative. I can do this when merging it need be.
2. You need to be covered by an appropriate participation agreement which applies to at least the URL workstream.

[rubycon]

@jeremyroman I updated my PR title and submitted a participant agreement, waiting for its validation.

[jeremyroman]

The WHATWG page says:

@rubycon has signed up to participate as an individual, but has not yet been verified. Hold tight while we sort this out on our end!

@annevk do you know what need to be done for it to be "verified"? I'm not super familiar with how individual participation works.

[domenic]

https://github.com/whatwg/participate.whatwg.org#process-for-editors :)

[jeremyroman]

Gotcha; done. (Was that linked from the check status page? If so, I missed it.)