fix Last-Modified header

elnode--rfc1123-date was incorrectly sending the month number as day
number, which leads to incorrect caching in browsers.

The defect went as such:

(elnode--rfc1123-date (list 20986 3923 0 0)) Thu, 08 Aug 2013 07:33:39 GMT

(format-time-string "%Y-%m-%dT%T%z" (list 20986 3923 0 0) 2013-08-01T09:33:39+0200