obtainEditableInstance wipes draft changes

When calling obtainEditableInsance on the workflow of a document as the holder of that document, and the owner has already saved changes to the draft version, then these changes are wipes out because the "content" of the preview variant get copied over the content of the draft variant.

This is a problem for Visual Editing: when switching from the content perspective to visual editing of the same document, the changes made in the content perspective are reverted.