Use find_page as a before filter and use @page.

The method find_page does not always returns the page. It returns he params hash in the case of hash assignation (preview existing pages).

This break previews view view_templates as object methods are called in RenderOptions::render_options_for_template.
Merge pull request #2218 from hooktstudios/preview-controller-fix

Use find_page as a before filter and use @page