Better deal with at url parameters that have weird values

If the `at` url parameter contains a value that is neither a date nor a timestamp integer then prevent the date() function to fail due to an illegal parameter type.

For example given `&at=xxx` this would happen before this patch.

Author: fiwswe

doku.php

@@ -64,7 +64,7 @@
     if ($date_parse) {
         $DATE_AT = $date_parse;
     } else { // check for UNIX Timestamp
-        $date_parse = @date('Ymd', $DATE_AT);
+        $date_parse = @date('Ymd', (int) $DATE_AT);
         if (!$date_parse || $date_parse === '19700101') {
             msg(sprintf($lang['unable_to_parse_date'], hsc($DATE_AT)));
             $DATE_AT = null;