fix: Update cash balance methods to no longer require nested ID.

[dcr-stripe]

Summary

Updates Customer.retrieve_cash_balance and Customer.update_cash_balance methods to no longer require a nested ID.

Previously you always had to call with nil as the second parameter (eg. retrieve_cash_balance('cus_123', nil)).
With this PR, the second parameter is nil by default.

Changes to retrieve_cash_balance

You can now use retrieve_cash_balance with the following call patterns:

1. Original pattern: retrieve_cash_balance(customer_id, nil[, opts])
2. New pattern: retrieve_cash_balance(customer_id[, opts])

Changes to update_cash_balance

You can now use update_cash_balance with the following call patterns:

1. Original pattern: update_cash_balance(customer_id, nil[, params[, opts]])
2. New pattern: update_cash_balance(customer_id) (admittedly this isn't very useful in isolation)

We raise an error if trying to do update_cash_balance(customer_id, some hash) since we don't want users to accidentally use this thinking the signature is update_cash_balance(customer_id, params). We unfortunately can't gracefully support both the legacy pattern and update_cash_balance(customer_id, params, opts) since we wouldn't be able to differentiate update_cash_balance(customer_id, nil, hash) (is has here a params or opts?).

Future major

In a future major, we could remove the nested_id nil parameter entirely and bring this back to a consistent syntax with our other APIs.

[richardm-stripe]

Thanks for the tests