Issue Overview

Gutenberg 2.7 & 2.8 does not assign new URL permalink, except at initial save.

Steps to Reproduce

1. Create post - use title "Post ABC"
2. Save. Permalink is automatically generated to ...post-abc
3. Edit title to "Post 123". Edit permalink to be empty.
4. Resave. Permalink should change to ...post-123, but it instead reverts to prior ...post-abc.