The API should allow updates that don't actually change anything. This allows
clients to, for example, accidentally send the same request twice without
encountering unexpected errors. This currently works for posts, terms, and
users, so this commit adds test cases accordingly.
See #38700 for issues preventing this from working for comments.
Fixes#38975.
git-svn-id: https://develop.svn.wordpress.org/trunk@39371 602fd350-edb4-49c9-b593-d223f7449a82
c716e604c5