Fix attachObject bug

Summary: so it was said in IRC and so it is true

Test Plan: saving maniphest tasks with custom fields no longer barfs

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8279

src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php

@@ -423,6 +423,7 @@ abstract class PhabricatorApplicationTransactionEditor
       $xaction->setAuthorPHID($actor->getPHID());
       $xaction->setContentSource($this->getContentSource());
       $xaction->attachViewer($this->getActor());
+      $xaction->attachObject($object);
     }
 
     $is_preview = $this->getIsPreview();