[w3c/payment-handler] Need to specify CanMakePaymentEvent.respondWith() behavior (#295)

We already added `CanMakePaymentEvent` to our spec but missed respondWith()'s behavior. In general, we understand how it works, but we need to define in more details in terms of spec. Otherwise, it might make some security holes.

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https://github.com/w3c/payment-handler/issues/295