invoice api should support sorting disputes by initiated_at

[kyungmin]

The API is throwing an error when fetching the disputes using /invoices/:invoice_id/disputes.

Request:

https://api.balancedpayments.com/invoices/IV4azZHI0CLnmHQBeGt7xI5q/disputes?limit=50&sort=initiated_at%2Cdesc&offset=0&type=debit&status%5Bin%5D=pending%2Cwon%2Clost%2Carbitration

Error:

description: "Unable to sort on unknown field "initiated_at" Your request id is OHM4eadba4c092211e4b88e02b12035401b."