Local autosaves: always set 'autosave = true' in the post data when triggering local autosave on form submit. Fixes cases where the form is submitted shortly after loading the Edit screen, local autosave hasn't run yet and there is no previous data to be merged. Fixes #23960.

git-svn-id: https://core.svn.wordpress.org/trunk@24328 1a063a9b-81f0-0310-95a4-ce76da25c4cd