Gocardless-pro

- Fix throwing error on empty 204 responses

- Added refund_funds_returned to payout item types
- Added funds_settlement to the mandate model

- Added request_pointer to validation errors

- Update pagination definition (non-functional change)

- Added a new field, links[creditor] to the scheme identifier create API
- Added a new API to set and retrieve negative balances
- Added a new bank_reference_prefix to the creditor API
- Removed the bank_redirect field from the institutions API
- Added an autocompletes_collect_bank_account field to the institutions API
- Removed the creditor API endpoint to apply scheme identifiers

- Added `country_code` as a required parameter to the List Institutions for Billing Request API.