Regression in lib/class-wp-json-posts.php where $post_fields['author'] is not set.

In b3b79d3c364abddb015a1b0feedacec2cf516532 this line was removed:
`$post_fields['author'] = $this->prepare_author( $post['post_author'] );`

This causes a bug when trying to update or insert a post if author is not explicitly set.