- Updates cron's sending queue worker to use model objects

- Adds new method to render newsletter to the newsletter model
- Adds new transient object to newsletter model that will hold temporary
  values (i.e., rendered body) when working with the model

lib/Cron/Workers/SendingQueue/Tasks/Shortcodes.php

@@ -6,9 +6,8 @@ use MailPoet\Newsletter\Shortcodes\Shortcodes as NewsletterShortcodes;
 if(!defined('ABSPATH')) exit;
 
 class Shortcodes {
-  static function process($content, array $newsletter, array $subscriber, array $queue) {
+  static function process($content, $newsletter, $subscriber, $queue) {
     $shortcodes = new NewsletterShortcodes($newsletter, $subscriber, $queue);
     return $shortcodes->replace($content);
   }
-}
-
+}