Fixed the return type of `DateObject`'s `DefaultValue` override.
Fixed the return type of `DateObject`'s `DefaultValue` override.

fholm/IronJS,xamarin/IronJS,fholm/IronJS,fholm/IronJS,hnafar/IronJS,hnafar/IronJS,xamarin/IronJS,hnafar/IronJS,xamarin/IronJS,hnafar/IronJS,xamarin/IronJS,hnafar/IronJS,fholm/IronJS