Fixed #10825: fixed the 'U' format code to dateformat (and the date/now...

Fixed #10825: fixed the 'U' format code to dateformat (and the date/now filter/tag). Thanks to gsong and mir.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10716 bcc190cf-cafb-0310-a4f2-bffc1f526a37