[w3c/browser-payment-api] canMakePayment() needs to match .show() (#535)

We need to make sure `canMakePayment()` matches show with respect to how .data is checked in `.show()`. We should make a note about IDL conversions that can take place there and be clear that IDL violations result on in "false". 

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https://github.com/w3c/browser-payment-api/issues/535