Format the credit card month and year as said in the Merchnat Warrior doc.

According the MerchantWarrior API documentation, Is expected that cardExpiryYear and cardExpiryMonth follow the conditions bellow:

__cardExpiryMonth__
Example: 05
Notes: This must be in MM format. The month must be zero-padded if it’s less than 10. Valid Length: 2 digits.

__cardExpiryYear__
Example: 13
Notes: This must be in YY format. 
Valid Length: 2 digits.