Flush only tags

[drymek]

This pull request solve issue:

$user = $this->getUser();
$taggableRelation = new TaggableRelation();
$user->addTaggableRelation($taggableRelation);

$form = $this->createForm(new UserType(), $user);
$form->handleRequest($request); // form transformer is using loadOrCreateTags -> flush

this causes flush on user entity as well (without validation).