owimpute: Do not copy methods into variable_methods

The learner input requires that the single instance of
`impute.Model` be modified in place for the changes to
take effect.

Author: ales-erjavec

Orange/widgets/data/owimpute.py

@@ -512,7 +512,7 @@ def set_method_for_indexes(self, indexes, method_index):
                 method = impute.Default(default=value)
                 self.variable_methods[index.row()] = method
         else:
-            method = self.methods[method_index].copy()
+            method = self.methods[method_index]
             for index in indexes:
                 self.variable_methods[index.row()] = method