Support auto-draft status in the API #2737

Splitting this out from #2629 where it was proposed:

In WP-Admin, auto-draft is used to account for posts that are not yet “fully” drafted, and therefore disables e.g. revisions. In #2629 @adamsilverstein proposed that the API support the auto-draft status, to permit a client to set a post into that state to permit it to be updated without generating revisions (and to support all valid statuses).