PurchaseRequest/PurchaseResponse: Changes for when sequenceType is recurring

[jnvsor]

PurchaseRequest: With a recurring payment (Using mandates with sequenceType recurring) you are not required to send returnUrl

PurchaseResponse: In a recurring payment the API can return status paid immediately so isSuccessful shouldn't always return false and the comment here is incorrect:

When you do a purchase the request is never successful because you need to redirect off-site to complete the purchase.