Unable to delete private properties via openidm.update()

Changes introduced by OPENIDM-13983 ensure that privately scoped properties are not inadvertently deleted from the target object when updates occur via REST.  This is important because our REST contract dictates that a READ of an object followed by an UPDATE (without modification) should not result in any changes to the persistent state of the object.

However, in the case of internal reads and updates via calls to openidm.read() and openidm.update() the above content filtering does not apply.  Calls to openidm.read() do return all properties irrespective of whether they are privately scoped.  Thus calls to openidm.update() which intentionally omit privately scoped properties should result in the removal of the property. This is currently not the case and therefore private properties can only be removed via PATCH operations.

Internal calls to openidm.update() should allow for the deletion of privately scoped properties.