`json_encode()` returns different results for non UTF-8 strings in PHP 5.5+, versus earlier versions of PHP.

This fixes the unit tests that fail in earlier versions, see #30471 for fixing this globally in `wp_json_encode()`.


git-svn-id: https://develop.svn.wordpress.org/trunk@30534 602fd350-edb4-49c9-b593-d223f7449a82