Invalidate cache header only for published posts

Closes #1563

- Add new updatedAttributes() functionality to base models
- Update Post.edit(...) to pass along _updatedAttributes values
- Update Post.delete to set statusChanged to true
- Add checking for statusChanged to cacheInvalidationHeader()
- Update route tests that check for cache invalidation header
Merge pull request #2632 from jgable/invalidateCache

Invalidate cache header only for published posts