Maintain compatibility with earlier nil-values fix from main (#1136)

An earlier commit on main (#d5422b46 in PR #1136) fixed a subtle
issue when overriding default values with nil, by explicitly setting
`.created_at` and other attributes from the existing item when it was
being replaced. This caused issues on this PostgreSQL branch after
rebasing, as ActiveRecord behaves more as expected with respect to
`created_at` and therefore the line creating a local `created_at`
variable had been removed.

This commit reintroduces that variable, and tests now pass again.

app/models/content_item.rb

@@ -21,6 +21,10 @@ def self.create_or_replace(base_path, attributes, log_entry)
 
     result = previous_item ? :replaced : :created
 
+    # This doesn't seem to get set correctly on an upsert so this is to
+    # maintain it
+    created_at = previous_item ? previous_item.created_at : Time.zone.now.utc
+
     # This awkward construction is necessary to maintain the required behaviour -
     # a content item sent to Content Store is a complete entity (as defined in a schema)
     # and no-remnants of the item it replaces should remain.