[HOT FIX] Add the missing attribute to Invoice model
[HOT FIX] Add the missing attribute to Invoice model

Currently our invoicing background worker is constantly failing because of `NoMethodError: undefined method `cis_deduction='` error.

Xero API is incredibly hard to work with (insane security measures, outdated docs, not working properly API Previewer, etc). So proper test coverage is postponed because more time is required to reproduce the failure and write tests. Meanwhile the issue on production must be fixed as soon as possible.

It seems like Xero API has changed the response structure and now our client must be adapted to that.

gohiring/xero,gohiring/xero