Improve the way we can use payment method specific payment requests

[Viincenttt]

Suggestion to improve issue mentioned in #380

Example code:

public void CreatePaymentRequest(string paymentMethod, Type expectedType) {
        var amount = new Amount(Currency.EUR, 50m);
        var description = "my-description";
        var paymentRequest = new PaymentRequest() {
            Amount = amount,
            Description = description
        };
        switch (paymentMethod) {
            case PaymentMethod.CreditCard:
                paymentRequest = new CreditCardPaymentRequest(paymentRequest) {
                    CardToken = "card-token"
                };
                break;
            case PaymentMethod.Ideal:
                paymentRequest = new IdealPaymentRequest(paymentRequest) {
                    Issuer = "ideal_issuer"
                };
                break;
        }