Moved from #4010 h/t @bradyvercher.

Steps to reproduce:

1. Insert a latest posts block.
2. Select the center alignment option in the block toolbar.
3. Deselect the center alignment option in the block toolbar.
4. Publish the post.

Result: the class is aligncenter even though alignment was deselected.

^{Seen at http://alittletestblog.com/2018/07/11/latest-posts-block/ running WordPress 4.9.7 and Gutenberg 3.2.0 using Firefox 61.0.1 on macOS 10.13.5.}