Reload the lastModified after an implicit save via HasManyAssociation#concat
Fixes an error like this:
    RuntimeError:
           ERR STATUS: 409 Could not remove triple containing predicate
           http://fedora.info/definitions/v4/repository#lastModified to
           node
           /test/1a/de/75/90/1ade7590-02aa-4e63-8c60-e5c34e11c170...
           when updating
           http://localhost:8983/fedora/rest/test/1a/de/75/90/1ade7590-02aa-4e63-8c60-e5c34e11c170.