Updates to credit memos feature

[bhelx]

This contains 2 breaking changes:

1. Invoice#mark_failed now returns InvoiceCollection

mark_failed no longer reloads the invoice with the response returning true or false, it returns either an InvoiceCollection if failable and request is successful, it returns false if invoice cannot be marked failed. To keep functionality, take the charge_invoice of the returned collection:

invoice = Recurly::Invoice.find('1001')

failed_collection = invoice.mark_failed
if failed_collection
  invoice = failed_collection.charge_invoice
end

2. Subscription previews and InvoiceCollection

Subscription previews and preview changes now return InvoiceCollections rather than Invoice. Utilize the charge_invoice to keep functionality the same:

subscription.preview

# Change
invoice = subscription.invoice
# To
invoice = subscription.invoice_collection.charge_invoice

[csmb]

Looks good to me!