DEB-6964 Add direct calls into Backbone.Model.set where possible.

@sergiopereiraTT I added logic to try to short-circuit Backbone.NestedModel.prototype.set with Backbone.Model.prototype.set where possible.  This avoids some inefficiencies in NestedModel's set code.  I ran NestedModel's test suite and everything passes.  Regarding code style, I tried to follow NestedModel's conventions.  Please let me know if you'd like me to change anything.