use getTime() to better support firefox

maybe you already know it (and you intentionally didn't change it), but there's a ["bug"](https://bugzilla.mozilla.org/show_bug.cgi?id=810973) in firefox where:

```javascript
t.same(new Date(new Date('2011-09-13T17:09:30.909Z')) , new Date('2011-09-13T17:09:30.000Z'));
```
>(Date constructor rounds off milliseconds when passed a Date object)


I was [using traverse](https://github.com/iamdoron/datify/blob/master/index.js) and I had trouble passing firefox/19 tests (and it's still an issue in 24).

As for `new Date(src.getTime())`, I saw it working in latest safari/firefox/node .