Invalidate cache header only for published posts

Closes #1563

- ~~Add check for post.status === 'published'~~
- 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

I have a couple open questions about whether I'm even solving the right problem described in #1563, but just in case this is all that is necessary I'm gonna put it up here.