JulianDate.fromIso8601 throws error

``` javascript
var date = new Cesium.JulianDate()
var isoDate = Cesium.JulianDate.toIso8601(date) // "-4713-11-24T12:00:00Z"
Cesium.JulianDate.fromIso8601(isoDate) // throws DeveloperError "Invalid ISO 8601 date."
```

Start by running the JulianDate.toIso8601 and JulianDate.fromIso8601 example from the issue, then locate the implementations of those entry points. Check why the date produced by toIso8601 is rejected by fromIso8601; done means the shown round trip no longer throws DeveloperError.

Written by the indexing model from the issue text.