If a published date is not manually specified, but it's in the future, don't overwrite it.

Fixes scheduled posts getting reset to the current published date if you have to make an edit after scheduling them.

Related to habari/habari#395.
Publish scheduled posts if their dates are changed into the past.

This is the opposite of the logic which changes a published post to scheduled if the date is in the future.

This could happen if you publish a post with a date in the future (so it becomes scheduled), then edit again and move the date into the past and save -- it would remain 'scheduled', when it should be made 'published'.