This ensures all the column titles, not just the Comments column one, are
"clean" when later reused for the Screen Options checkboxes. Also introduces
some consistency with what is already done in WP_List_Table.
Fixes#41261.
Built from https://develop.svn.wordpress.org/trunk@41148
git-svn-id: http://core.svn.wordpress.org/trunk@40988 1a063a9b-81f0-0310-95a4-ce76da25c4cd