Check if a page has actually been modified before calling api->save

[sy238-unikent]

No point making needless API requests

... although it might be that another user has changed the page, and actually we want to overwrite that with what we had saved...

Might be better to make the API call after all and let the API decide if it needs to do anything... possibly return a different status code if nothing to update, so Editor can decide whether or not to show "saved" dialog.