[w3c/payment-request] PaymentMethodData should be converted at construction time (#813)

Related to #753 

Given the following: 

```JS
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. 



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