JulianDate.toIso8601 builds a non compliant string with very small milliseconds

[jfayot]

console.log(Cesium.JulianDate.toIso8601(Cesium.JulianDate.fromIso8601("2011-03-25T16:00:00.00000001Z"))

outputs:

2011-03-25T16:00:00.10000803740695119e-7Z

Which is not ISO8601 compliant !

In JulianDate.js, line 787, the comment says:

//Forces milliseconds into a number with at least 3 digits to whatever the default toString() precision is.

but toString will automatically convert the milliseconds into scientific notation when lower than 1e-6. number.toFixed is not appropriate neither, something more sophisticated needed here...