time.Time fields may be null

[tarasyarema]

Describe the bug

Some time fields (the Invoice.DueAt for instance) may return a null value from the API but the client does not support pointer time fields, hence the set value on the resource is a zero value time.Time.

To Reproduce

Create an Invoice with no DueAt and fetch it via the API client in Go.

Expected behavior

DueAt to be a *time.Time, hence when a null value is fetched it's a nil and not a t time.Time with t.IsZero() == true.

Your Environment

• Which version of this library are you using? github.com/recurly/recurly-client-go/v3 v3.13.0
• Which version of golang are you using? go version go1.15 darwin/amd64