Closing a blog post always states 'Document has been modified' on closing, even if nothing changed

Reporting as Essentials issues as this seems specific to blog post documents

Steps to reproduce:
1) Create a new project and add the blog feature. Rebuld
2) Login CMS and go to /content/documents/essentialstest/blog/2015/04/first-blog-post
3) Click Edit. Do not change anything
4) Close the tab by clicking (X)
Expected: Document closes without dialog
Actual: Modal dialog: Document has been modified. Save changes?

Reproduced with a new blog post and after saving first, before (re) opening
Not reproduced with other document types: Author, Event, FAQ