Improve compliance of CreateAgreement with API spec

[RobinTTY]

Currently the class CreateAgreementRequest, which is used as a payload to create an end user agreement, has the required properties MaxHistoricalDays, AccessValidForDays, AccessScope and InstitutionId. The GoCardless documentation mentions that MaxHistoricalDays, AccessValidForDays and AccessScope are optional when sending the request. The class should be changed to respect the API spec and tests added to see if the actual behavior is as described in the documentation.