Anonymizer within extensions

[flo-dup]

Do you want to request a feature or report a bug? Bug

What is the current behavior? Anonymizer is not used with extensions serialization.

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

1. Create or import a network with an extension which contains id information, for instance BaseVoltageMapping
2. Export with anonymizer
3. Import (or edit) the generated file and look at the ids within the extension tag

What is the expected behavior? Ids are anonymized

What is the motivation / use case for changing the behavior? The extensions are of no use if the ids do not refer to anything.