MDL-35966 Assignment - Localisation issue - use userdate not strftime

This is causing a database error (reported on MySQL) when strftime returns
non-utf8 characters.  This occurs when sending assignment notifications.

This was reported and a solution provided by Chien Wen-Chang(簡文章).

mod/assign/locallib.php

@@ -3182,7 +3182,7 @@ public static function send_assignment_notification($userfrom, $userto, $message
         }
         $info->assignment = format_string($assignmentname,true, array('context'=>$context));
         $info->url = $CFG->wwwroot.'/mod/assign/view.php?id='.$coursemodule->id;
-        $info->timeupdated = strftime('%c',$updatetime);
+        $info->timeupdated = userdate($updatetime, get_string('strftimerecentfull'));
 
         $postsubject = get_string($messagetype . 'small', 'assign', $info);
         $posttext = self::format_notification_message_text($messagetype, $info, $course, $context, $modulename, $assignmentname);