Updating card_expiration_date format to match current API

[vitork]

Pagar.me now accepts card_expiration_date in the format MMYY or MMYYYY.
Ref.: https://docs.pagar.me/reference#criar-transacao

Fixes #2534

[bpollack]

@vitork If this is still relevant to you, would you mind adding at least unit tests, and ideally remote tests?

[vitork]

Hey @bpollack!

No, this is not relevant anymore. The gateway API has changed a lot and I believe the whole integration would need to be rebuilt. For that reason I've decided not to use Active Merchant for this specific gateway and to implement the integration manually since I'm using just some basic endpoints.

Unfortunately I don't have the time to implement the necessary changes for the API to work again at this moment, but I'll be happy to help in the future if the integration is still outdated.

If you don't mind, I'm closing this PR since the code changes here would not solve the problem at all. As for the issue #2534, I'm not sure if we should close it or leave it opened to sinalize that the gateway integration is not working.

Thanks!