Include ScheduledAt in Refs.

[boscar]

Right now, scheduledAt is recieved when fetching the api as a unix timestamp. This is not parsed to the Ref DateTime object. It would be nice if scheduledAt would be included in the Ref object.

[benembery]

@boscar this is supported in the newer .NET standard library. You could PR it back into this library if you can't upgrade.
https://github.com/adaptivewebworks/prismicio-netstandard-kit/blob/master/src/prismicio.AspNetCore/Ref.cs