SklImpute: use ReplaceUnknowns directly

Orange/preprocess/preprocess.py

@@ -163,7 +163,7 @@ def __call__(self, data):
         # drop the ones which do not have valid `imputer.statistics_`
         # (i.e. all NaN columns). `sklearn.preprocessing.Imputer` already
         # drops them from the transformed X.
-        features = [impute.Average()(data, var, value)
+        features = [var.copy(compute_value=impute.ReplaceUnknowns(var, value))
                     for var, value in zip(data.domain.attributes,
                                           imputer.statistics_)
                     if not np.isnan(value)]