Updating an instance fails

Fixes #1082

This is related to the workaround put in for RAILS bug
https://github.com/rails/rails/issues/17466

The code was expecting all values to be present in the
ae_instance.ae_values, the import/export only add non-empty values.
The UI seems to add all values even if they are empty.
So in a new database after the import/export only non empty values are
present as soon as you save it in the DB all values get added by the UI.
If this is a issue we can open a separate git issue for it.