* reverseproxy: Sync up `handleUpgradeResponse` with stdlib
I had left this as a TODO for when we bump to minimum 1.17, but I should've realized it was under `internal` so it couldn't be used directly.
Copied the functions we needed for parity. Hopefully this is ok!
* Add tests and fix godoc comments
Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>
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Francis Lavoie