store planned data source state when deferring

This copies the behavior of resources, so that there is a placeholder
state available for planning.

terraform/eval_read_data_plan.go

@@ -88,7 +88,7 @@ func (n *evalReadDataPlan) Eval(ctx EvalContext) (interface{}, error) {
 		}
 
 		*n.State = &states.ResourceInstanceObject{
-			Value:  cty.NullVal(objTy),
+			Value:  proposedNewVal,
 			Status: states.ObjectPlanned,
 		}