- Patch #327269 by c960657: when drupal_page_cache_header() compares the...

- Patch #327269 by c960657: when drupal_page_cache_header() compares the client's If-Modified-Since header to $cache->created, it assumed a certain date format.  However, HTTP/1.1 allows several variations of the date format, i.e. the same time may be represented in slightly different ways. If the client sends the date in a different format than the one generated by Drupal, it would never receive a 304 Not Modified response.  Also added a good amount of tests for the drupal_page_cache_header() code.