Fix email template creation date not being persisted

The database schema from https://github.com/magento/magento2/blob/develop/app/code/Magento/Email/sql/email_setup/install-2.0.0.php#L62-L67 uses the column `added_at` for storing the date the email template creation date. When `setCreatedAt` is used, that date is not persisted to the database, leaving the column `NULL` in the database.

Author: kabel

app/code/Magento/Email/Model/Resource/Template.php

@@ -99,7 +99,7 @@ public function checkCodeUsage(\Magento\Email\Model\Template $template)
     protected function _beforeSave(AbstractModel $object)
     {
         if ($object->isObjectNew()) {
-            $object->setCreatedAt($this->dateTime->formatDate(true));
+            $object->setAddedAt($this->dateTime->formatDate(true));
         }
         $object->setModifiedAt($this->dateTime->formatDate(true));
         $object->setTemplateType((int)$object->getTemplateType());