Use the same timestamp for etag as for last-modified

According to MDN, the last-modified header is a fallback to the etag.
Thus, we should be using the same value explicitly. Prior to this
commit, the value was implicitly the same, because under the hood,
Alchemy used the `published_at` timestamp.

With this commit, it's explicitly the same and allows Alchemy to
implement a `last_modified_at` method that will be respected.

app/controllers/alchemy/json_api/pages_controller.rb

@@ -10,9 +10,12 @@ def index
 
         jsonapi_filter(page_scope, allowed) do |filtered_pages|
           @pages = filtered_pages.result
+          last_modified_page = @pages.select { |page| last_modified_for(page) }.max_by { |page| last_modified_for(page) }
+          last_modified_at = last_modified_for(last_modified_page) || Time.current
+          latest_etag = last_modified_page&.cache_key_with_version || Time.current.to_s
           if !@pages.all?(&:cache_page?)
             render_pages_json(allowed) && return
-          elsif stale?(last_modified: @pages.maximum(:published_at), etag: @pages.max_by(&:cache_key_with_version)&.cache_key_with_version)
+          elsif stale?(last_modified: last_modified_at, etag: latest_etag)
             render_pages_json(allowed)
           end
         end
@@ -61,7 +64,8 @@ def load_page_for_cache_key
       end
 
       def last_modified_for(page)
-        page.published_at
+        return unless page
+        page.respond_to?(:last_modified_at) ? page.last_modified_at : page.published_at
       end
 
       def jsonapi_meta(pages)

spec/requests/alchemy/json_api/pages_spec.rb

@@ -43,7 +43,11 @@
 
         it "sets public cache headers" do
           get alchemy_json_api.page_path(page)
-          expect(response.headers["Last-Modified"]).to eq(page.published_at.utc.httpdate)
+          if Alchemy.gem_version >= Gem::Version.new("7.2.0")
+            expect(response.headers["Last-Modified"]).to eq(page.public_version.updated_at.utc.httpdate)
+          else
+            expect(response.headers["Last-Modified"]).to eq(page.published_at.utc.httpdate)
+          end
           expect(response.headers["ETag"]).to eq('W/"0741fe32d81bfdabfeb47d9939c5f6b7"')
           expect(response.headers["Cache-Control"]).to eq("max-age=10800, public, must-revalidate")
         end