Resolve #3589076 "Detect conflicts during autosave polling" (!1107) · Merge requests · project / canvas

[Description of changes]

Testing instructions

Create Page entity in Canvas, make a change, publish the changes
Modify same Page entity, but do not publish your changes, observe auto-save polling (HTTP GET to /canvas/api/v0/auto-saves/pending) respond with HTTP 200
Outside of Canvas resave the Page entity (using drush esav canvas_page <entity id> -y should work just fine)
Observe next Canvas auto-save poll (HTTP GET to /canvas/api/v0/auto-saves/pending) return HTTP 409