A `post_format` not used by the current theme, but supported by core is not a wrong/broken piece of information. It's just not used at this point in time. Therefore we should allow setting and retrieving any of the standard post formats supported in core, even if the current theme doesn't use them.
After this commit, a post's `format` value can survive a round trip through the API, which is a good general design principle for an API.
Props JPry, iseulde, davidakennedy, Drivingralle.
Fixes#39232.
git-svn-id: https://develop.svn.wordpress.org/trunk@40120 602fd350-edb4-49c9-b593-d223f7449a82
162e1a6f2a