Uniformize adding parameter child nodes

openfisca_core/parameters.py

@@ -418,7 +418,7 @@ def merge(self, other):
         In case of child name conflict, the other node child will replace the current node child.
         """
         for child_name, child in other.children.items():
-            self.children[child_name] = child
+            self.add_child(child_name, child)
 
     def add_child(self, name, child):
         """