Restore the functionality of date_i18n to pre [17747] so that is works without...

Restore the functionality of date_i18n to pre [17747] so that is works without a provided timestamp again. Fixes #17278.

git-svn-id: https://core.svn.wordpress.org/trunk@17841 1a063a9b-81f0-0310-95a4-ce76da25c4cd