Improvement for published posts with no published_at

This is not a fix, merely an improvement for 0.4.1.

I think the real issue stems from the problems documented in #1655 and will require major refactoring work.

Here we at least make sure that the published date gets set on a future save.

issue #2015

- this is another little workaround / improvement to try to reduce the number of people who end up with a published post with no published_at set
- I assume we need to complete #1655 to fix this properly