Add the reference date to the model properties and the getMethod

[lionslair]

The package is missing the reference field from the docs. I have added to the payroll and can confirm this works
https://developer.xero.com/documentation/api/payrollau/payrollcalendars/#get-payrollcalendars

[
  {
    "Name": "Fortnightly",
    "StartDate": "2021-09-01",
    "PaymentDate": "2021-09-06",
    "CalendarType": "FORTNIGHTLY",
    "ReferenceDate": "2021-09-01",
    "UpdatedDateUTC": "2021-09-30",
    "PayrollCalendarID": "05682961-e905-47fc-b3be-e9c2b698388e"
  },
  {
    "Name": "Weekly",
    "StartDate": "2021-09-08",
    "PaymentDate": "2021-09-13",
    "CalendarType": "WEEKLY",
    "ReferenceDate": "2021-09-01",
    "UpdatedDateUTC": "2021-10-06",
    "PayrollCalendarID": "7a316b34-c1d9-40a7-bae0-10fcde430171"
  }
]

The ReferenceDate field is now returned