Rollup with Moment duration or certain duration factory methods generates error

The happy path works:

```
series.fixedWindowRollup({
window(duration('1h')),
aggregation: {v_sum: ['v', sum()]}
})
```

However, the following does not work, and generates `TypeError: Cannot read property 'mid' of undefined`:

```
series.fixedWindowRollup({
window(duration(moment.duration(1, 'h'))),
aggregation: {v_sum: ['v', sum()]}
})
```

Other variants have issues too:

```
duration(moment.duration(1, 'h').asMilliseconds())
```

But, if you hack the `_string` property, it works!

```
const d = duration(moment.duration(1, 'h').asMilliseconds())
d._string = '1h' // Normally is PT1H when moment is used
```

So it seems that rollups are sensitive to the string property?