Method ::buildCreditCardForm doesn't use padded months for values.

Currently the method that builds the credit card form uses non-padding values for the select list. However it uses them for the display. Gateways will fail if passed the non-padded value.

Proposed change

    $months = [];
    for ($i = 1; $i < 13; $i++) {
      $month = str_pad($i, 2, '0', STR_PAD_LEFT);
      $months[$month] = $month;
    }