DurationFormat resolvedOptions() output should always include fractionalDigits

[gibson042]

Objects returned from resolvedOptions() sometimes omit fields, but generally only when they have no meaningful interpretation (e.g., NumberFormat {minimum,maximum}{Fraction,Significant}Digits or Collator numeric/caseFirst when those are not relevant extension keys).

But DurationFormat omits fractionalDigits whenever input options do not define a numeric value for it, which seems odd. Rather than having Object.getOwnPropertyDescriptor(new Intl.DurationFormat().resolvedOptions(), "fractionalDigits") return undefined (implicitly propagating a value that might be defined on Object.prototype), I'd like to see it return { value: undefined, writable: true, enumerable: true, configurable: true }, such that the shape of returned objects is always consistent.