Builtins: Default values in date/time setters and the subclassing protocol

[lth]

spec/library/Date.es: ES3 has language about supplying default values for the date setters (setSeconds etc) that is imprecise, it says that eg if ms is missing then the value will be as if getMilliseconds was called. There's no implication about whether it actually is called.

For ES4 I propose that we clean this up and make it into a proper OO protocol s.t. the default value is supplied by an actual call to intrinsic::getMilliseconds (which is the canonical method here); the user may override that function to get new behavior. But note that this places constraints on how implementations handle date objects.