`datetime` and `timestamp` array attributes are saving correctly when `model` is aware about `time_zone`

fixes #13402

In PG adapter, when attributes are of `datetime` or `timestamp` type and can store `array`, then they are not saving properly if `Model.time_zone_aware_attributes = true`. [Here is the gist](https://gist.github.com/kuldeepaggarwal/8306031)