Use a float for last_changed microtime cache values.

microtime() by default returns a string with a space, which isn't allowed for keys in some cache backends.

props _jameslee, drozdz.
fixes #27000. see #23448.

Built from https://develop.svn.wordpress.org/trunk@27115


git-svn-id: http://core.svn.wordpress.org/trunk@26982 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix typo in wp_set_auth_cookie() description.

props drozdz.
fixes #27046.
Built from https://develop.svn.wordpress.org/trunk@27116


git-svn-id: http://core.svn.wordpress.org/trunk@26983 1a063a9b-81f0-0310-95a4-ce76da25c4cd