Add possibility to preselect payment method and some other tweaks

[macolo]

This allows a client to preselect a payment method. SIX Saferpay then does not show the payment selection screen but continues straight away with the preselected payment method according to:

Used to restrict the means of payment which are available to the payer for this transaction. If only one payment method id is set, the payment selection step will be skipped.
Possible values: ALIPAY, AMEX, BANCONTACT, BONUS, DINERS, DIRECTDEBIT, EPRZELEWY, EPS, GIROPAY, IDEAL, INVOICE, JCB, MAESTRO, MASTERCARD, MYONE, PAYPAL, PAYDIREKT, POSTCARD, POSTFINANCE, SAFERPAYTEST, SOFORT, TWINT, UNIONPAY, VISA, VPAY, KLARNA.
Example: ["VISA", "MASTERCARD"]

http://saferpay.github.io/jsonapi/#Payment_v1_Transaction_Initialize