Fix/timestamp

firefox behaves differently when you `new Date(new Date)`,
 it must be `new Date(d.getTime())` or `new Date(d.toISOString())`:

![screen shot 2014-07-16 at 10 42 36 am](https://cloud.githubusercontent.com/assets/1661587/3595918/d0599a96-0cbc-11e4-86f3-d11962075e42.png)

needed for https://github.com/segmentio/analytics.js/pull/386

cc @calvinfo