Fix update of multivalued hierarchical vocabulary field

Steps to reproduce:

1. Log in at https://nightly.nuxeo.com/nuxeo/
2. Navigate to a document, https://nightly.nuxeo.com/nuxeo/ui/#!/browse/default-domain/workspaces/Sample%20Content/PDF%20and%20Office%20Documents/Sample%20PDF%20File.pdf for example
3. Edit the document
4. Choose "Art history" for the Subjects field
5. Save
6. Edit the document a second time
7. Choose "Literature" for the Subjects field
8. Save
9. Run the JSON export
10. Observe that the parent is missing for the "Art History" value as in this excerpt

        "dc:subjects": [
          "art history",
          "art/literature"
        ],
    

    => we have "art history" instead of "art/art history"

Expected behavior:
Editing the document should keep the correct values.