[DRAFT] Allows native WebSocket implementations to use close codes from RFC 6455

[kpsroka]

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[kpsroka]

Hi @brianquinlan,
This is the approach I want to take wrt #1294. Let me know if that looks fine to you, and I'll continue making this change prod-ready.

[brianquinlan]

Looks good to me! Let me know if I should re-run the checks.

[brianquinlan]

Maybe change must => will because this is meant to be read by a WebSocket user not implementer ;-)