Verify that updated_at is mapped correctly

[henrik]

Namely: say the mapper copies timestamp x from record to entity. Later, at timestamp y, we modify it and ask the mapper to persist it from entity to record. We don't want the old timestamp x to be persisted. Not sure if it will be currently.

We could either ignore the passed in "updated_at" and let Rails do its thing, or perhaps we could have the entity or mapper set the "updated_at" value to Time.now when persisting, unless the value was manually set. Or something.