Add the keys to the UpdatePaymentMethodRequest data

[MattyRad]

There's quite a few additional keys that Braintree accepts as part of its Update payment method endpoint (https://developers.braintreepayments.com/reference/request/payment-method/update/php#arg.token). I'm adding cardholderName and expirationDate because those are keys that I need at the moment; I'd like to keep the LoC low for this first PR, but I can also add the other missing keys if necessary.

• [x] Fork the project.
• [x] Make your feature addition or bug fix.
• [x] Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Note: I updated existing tests, but if you prefer new tests to get written I can do that as well
• [x] Commit just the modifications, do not mess with the composer.json or CHANGELOG.md files.
• [x] Ensure your code is nicely formatted in the PSR-2 style and that all tests pass.
• [x] Send the pull request.
• [x] Check that the Travis CI build passed. If not, rinse and repeat.

[kbussche]

Any chance we can get this added to the main branch?