Follow the pattern of wp-admin a bit more for metaWeblog.newPost by using...

Follow the pattern of wp-admin a bit more for metaWeblog.newPost by using get_default_post_to_edit() to generate a post ID before the post is actually published.

Fixes #18812
Props koke, nacin


git-svn-id: https://core.svn.wordpress.org/trunk@18933 1a063a9b-81f0-0310-95a4-ce76da25c4cd