Matches relevant fix for Twenty Fourteen, Fifteen, and Seventeen: https://core.trac.wordpress.org/ticket/39531
For the post preview to show the 2nd category I added, after saving the post.
The category is not shown in the preview. And because of how twentysixteen_categorized_blog() returns false for 1 or fewer categories, in this case it won't show the new category.
Fix category count when previewing with only 1 category