Set newsletter's ID when sending preview or actual newsletter

[MAILPOET-1882]

lib/API/JSON/v1/Newsletters.php

@@ -377,6 +377,7 @@ class Newsletters extends APIEndpoint {
         $rendered_newsletter['body']['html'],
         $rendered_newsletter['body']['text']
       ) = explode($divider, $shortcodes->replace($body));
+      $rendered_newsletter['id'] = $newsletter->id;
 
       try {
         $mailer = (!empty($data['mailer'])) ?

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

@@ -156,6 +156,7 @@ class Newsletter {
     }
     $prepared_newsletter = Helpers::splitObject($prepared_newsletter);
     return array(
+      'id' => $newsletter->id,
       'subject' => $prepared_newsletter[0],
       'body' => array(
         'html' => $prepared_newsletter[1],