Fix #3419: Deleting a post doesn't clear parent's "parent" status.

Bug: when deleting a child post and the "Move favorites to parent?" option is
set, the parent's has_active_children flag is not cleared.

`give_favorites_to_parent` moves the votes, and moving the votes has the
side effect of reloading the post (to get the new score). But reloading
the post wipes out the is_deleted_changed? flag, which is used by `update_parent_on_save`.

Fix: update the `is_deleted` flag *before* moving favorites, so that the
`update_parent_on_save` callback runs before `give_favorite_to_parent` runs.

test/unit/post_test.rb

@@ -353,6 +353,17 @@ class PostTest < ActiveSupport::TestCase
           p1.reload
           assert(p1.has_children?, "Parent should have children")
         end
+
+        should "clear the has_active_children flag when the 'move favorites' option is set" do
+          user = FactoryGirl.create(:gold_user)
+          p1 = FactoryGirl.create(:post)
+          c1 = FactoryGirl.create(:post, :parent_id => p1.id)
+          c1.add_favorite!(user)
+
+          assert_equal(true, p1.reload.has_active_children?)
+          c1.delete!("test", :move_favorites => true)
+          assert_equal(false, p1.reload.has_active_children?)
+        end
       end
 
       context "one child" do