Timestamps may have 6 numbers for milliseconds instead of 3 (as ISO specs)

Sometimes I find timestamps in different format, sometimes with 3 numbers for the milliseconds, and other times 6. Look at this order:

```json
{
"symbol": "AERI",
"submitted_at": "2019-11-06T22:24:44.596018Z",
"canceled_at": "2019-11-06T22:26:49.661Z",
"filled_avg_price": null,
"created_at": "2019-11-06T22:24:44.613307Z",
"replaced_at": null,
"expired_at": null,
"asset_id": "c9910301-f604-49ae-8363-7ac64fea99b3",
"type": "limit",
"updated_at": "2019-11-06T22:26:49.699988Z",
"stop_price": null,
"order_type": "limit",
"side": "buy",
"filled_qty": "0",
"replaces": null,
"asset_class": "us_equity",
"replaced_by": null,
"limit_price": "20.01",
"time_in_force": "day",
"extended_hours": true,
"qty": "96",
"filled_at": null,
"failed_at": null,
"status": "canceled"
}
```