use maybe_unserialize() in update and API checks, Tighten up the checks on...

use maybe_unserialize() in update and API checks, Tighten up the checks on expected return data to avoid processing invalid responses after change. See #19617

git-svn-id: https://core.svn.wordpress.org/trunk@19707 1a063a9b-81f0-0310-95a4-ce76da25c4cd