Do not set created_at/on timestamp for existing records

When `array_of_attributes` contains the value for `created_at`, `import` overwrites this value with `Time.now`. The expected behavior in Rails is to not overwrite the value if it's being passed in. The way I discovered this was I was passing in `WidgetModel#attributes` to the import method, and I observed the `created_at` values in the DB were being overwritten.