Minor updates dont update publish time

This changes the SpecialistDocumentDatabaseExporter and the IndexableFormatters to not change published_At times on minor updates. i.e. rummager's last_update date is set to retain the same last_update time and content-api is set to retain the same published_at. 

Trello ticket: https://trello.com/c/r9ga3xOa/438-minor-updates-should-not-update-updated-at