[w3c/payment-request] payment modifier brings error (#637)

why the following code:

```
function onBuyClicked() {
            if (!window.PaymentRequest) {
                // PaymentRequest API is not available. Forwarding to
                // legacy form based experience.
                location.href = '/checkout';
                return;
            }

            // Supported payment methods
            var supportedInstruments = [{
                supportedMethods: ['basic-card'],
                data: {
                    supportedNetworks: [
                        'visa', 'mastercard', 'amex', 'discover',
                        'diners', 'jcb', 'unionpay'
                    ]
                }
            }];

            // Checkout details
            var details = {
                displayItems: [{
                    label: 'Original donation amount',
                    amount: { currency: 'USD', value: '65.00' }
                }, {
                    label: 'Friends and family discount',
                    amount: { currency: 'USD', value: '-10.00' }
                }],
                total: {
                    label: 'Total due',
                    amount: { currency: 'USD', value : '55.00' }
                }
            };

            // Credit card incurs a $3.00 processing fee.
            const creditCardFee = {
                label: "Credit card processing fee",
                amount: { currency: "USD", value: "3.00" },
            };

            // Modifiers apply when the user chooses to pay with
            // a credit card.
            const modifiers = [
                {
                    additionalDisplayItems: [creditCardFee],
                    supportedMethods: "basic-card",
                    total: {
                        label: "Total due",
                        amount: { currency: "USD", value: "68.00" },
                    },
                    data: {
                        supportedTypes: "credit",
                    },
                },
            ];
            Object.assign(details, { modifiers });

            // 1. Create a `PaymentRequest` instance
            var request = new PaymentRequest(supportedInstruments, details);

            // 2. Show the native UI with `.show()`
            request.show();
        }
```

brings error:
```
test.html:509 Uncaught TypeError: Failed to construct 'PaymentRequest': The provided value cannot be converted to a sequence.
    at onBuyClicked (test.html:509)
    at HTMLButtonElement.onclick (test.html:427)
```

according to https://www.w3.org/TR/payment-request/#x2.2.2-conditional-modifications-to-payment-request it should works.


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