Credit card payments: cannot validate if the month format is numeric ('m', instead of default 'M')

If I set 'm Y' as the date format for "credit card expiration dates", CiviCRM systematically responds 'Credit card expiration date cannot be a past date.'

I tracked this down to CRM/Utils/Rule.php, currentDate().

See proposed patch.

Although I'm not sure if this should be fixed in the contribution form to generate an element always with the identifier "credit_card_exp_date[M]".