Support for Subscription first_billing_date

[mklnz]

I noticed when testing there is no support for first_billing_date for subscriptions during creation.
Braintree reference:
https://developers.braintreepayments.com/reference/request/subscription/create/ruby#set-start-date

Supposedly the billing_period_start_date should reflect and be calculated by first_billing_date and/or billing_day_of_month.

If anyone has a solution for testing this behaviour please let me know, otherwise I'll see if I can implement this in my spare time.

[mjc-gh]

We'd be interested in this behavior to. Perhaps I can put together a PR?

[birarda]

@mikeycgto if you can, it would be greatly appreciated.

High Fidelity doesn't really have the resources currently to devote the time this project deserves so any submitted pull requests are greatly appreciated!