Related to #753

Given the following:

var details = {
  total: {
    label: "Total",
    amount: {
      currency: "USD",
      value: "0.00",
    },
  },
};
new PaymentRequest(
  [
    { supportedMethods: "basic-card", data: { supportedNetworks: "☠️" } },
  ],
  details
);

Firefox currently throws, because it tries to do the IDL conversion:

TypeError: Fail to convert methodData.data to BasicCardRequest.

Chrome doesn't throw.

Per spec, Chrome is right... but it not doing the conversion seems not great.