Field Node.mtime only triggered by accident for django

[sphuber]

The ModelWrapper utility class ensures that a database model instance
is automatically saved if a model field is updated and the instance is
already stored. This prevents the implementation from having to call
save manually every time a field is updated. The Django implementation
uses the update_fields keyword of the save method, to update only
the field affected. However, this circumvents the auto_now of the
mtime field. This bug is currently not visible, because in the model
wrapper currently also always updates the node version after a model
field is updated of a stored instance. This, however, uses a normal
save and so does properly update the mtime. Once this updating of
the node version would have been removed, this bug will be revealed.