Adds the CardType field back into the form

[JarrydLong]

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Adds the CardType hidden field to the Braintree card fields form

Closing #3100 in favor of these changes

How to test the changes in this Pull Request:

1. Use a gateway that uses our own card field form such as Braintree or Auth.net
2. Fill out your card details and hit submit
3. Checkout should go through as expected

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?