[w3c/payment-handler] Use the correct origin serialization (#260)

Origin serialization is not defined by RFC6454 anymore, but instead by https://html.spec.whatwg.org/multipage/origin.html#ascii-serialisation-of-an-origin. That is the one that the rest of the browser uses, and should be used here too. (Note that it's not the Unicode serialization; no browser implements that.)

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https://github.com/w3c/payment-handler/issues/260