Normative: Revert to previous behaviour by setting fallback value for fractionalDigits to *undefined* (#150)

* changed fallback value for fractionalDigits to *undefined*, reverting to previous behaviour

* don't assert _v_ not undefined if _p_ is fractionalDigits

* protected FractionalDigits from assertion in resolvedOptions, rearranged table in resolvedOptions to put optional FractionalDigits last and non-optional NumberingSystem first, change to pass 0/9 rather than undefined/undefined into Intl.NumberFormat for minimumFractionDigits/maximumFractionDigits

* added note to properties of Intl.DurationFormat instances about [[FractionalDigits]] potentially being *undefined*

Author: ben-allen