Allow to merge Seo values from Controller

[Damien Alexandre]

At the moment, this works:

    public function showAction($slug, SeoManager $seoManager): Response
    {
        $seoManager->getSeo()->setTitle('coucou');

        return $this->render('page/show.html.twig');
    }

But only because the Seo object reference is kept, not because it's supported.

If the "mergeSeo" method was public I could swap the default Seo values easily.