Fix updating the public_on date on persisted pages

When updating a page's `public_on` date, we need to save the associated
public page as well. This commit accomplishes that.

Prior to this commit, we would set the `public_on` date on the public
version, but not persist it.

app/models/alchemy/page.rb

@@ -479,7 +479,11 @@ def public_on=(time)
         # Need to reset the public version on the instance so we do not need to reload
         self.public_version = nil
       elsif public_version
-        public_version.public_on = time
+        if persisted?
+          public_version.update!(public_on: time)
+        else
+          public_version.public_on = time
+        end
       elsif time.present?
         versions.build(public_on: time)
       end

spec/models/alchemy/page_spec.rb

@@ -1397,7 +1397,7 @@ def copy_children_to(new_parent)
     end
 
     describe "#public_on=" do
-      let(:time) { Time.now }
+      let(:time) { 1.hour.ago }
 
       subject { page.public_on = time }
 
@@ -1409,6 +1409,15 @@ def copy_children_to(new_parent)
           expect(page.public_version.public_on).to be_within(1.second).of(time)
         end
 
+        context "when the page is persisted" do
+          let(:page) { create(:alchemy_page, :public) }
+
+          it "sets public_on on the public version" do
+            page.update(public_on: time)
+            expect(page.reload.public_version.public_on).to be_within(1.second).of(time)
+          end
+        end
+
         context "and the time is nil" do
           let(:page) { build(:alchemy_page, :public) }
           let(:time) { nil }