Remove Page#published_at overwrite

We need the value from the database in all cases in order to be able
to use it as the last_modified header. The overwrite is not necessary
for the cache_key (ETag header) since it already takes the use case
for admins into account.

app/models/alchemy/page/page_natures.rb

@@ -117,15 +117,6 @@ def cache_key
         end
       end
 
-      # We use the published_at value for the cache_key.
-      #
-      # If no published_at value is set yet, i.e. because it was never published,
-      # we return the updated_at value.
-      #
-      def published_at
-        read_attribute(:published_at) || updated_at
-      end
-
       # Returns true if the page cache control headers should be set.
       #
       # == Disable Alchemy's page caching globally

spec/models/alchemy/page_spec.rb

@@ -1871,9 +1871,7 @@ class AnotherUrlPathClass; end
         let(:updated_at) { 3.days.ago }
         let(:page) { build_stubbed(:alchemy_page, published_at: nil, updated_at: updated_at) }
 
-        it "returns the updated_at value" do
-          expect(page.published_at).to be_within(1.second).of(updated_at)
-        end
+        it { expect(page.published_at).to be_nil }
       end
     end