(bug 46383) correct constructor fields for prototypes not using dv.util.inherit

Same as Ied01e553d16dd0b161fa97ea25d33d9f4efd0447 for Wikibase but for DataValues extensions.

Change-Id: Ib52694b932b5f9bdc0949b357960917c06593ef2
Merge "(bug 46383) correct constructor fields for prototypes not using dv.util.inherit"