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.


git-svn-id: https://develop.svn.wordpress.org/trunk@27115 602fd350-edb4-49c9-b593-d223f7449a82