allow select on nested iframe

When mail_view is embedded in an outer iframe, the multipart select  dropdown doesnt work. This fix corrects it.

I'm using the gem different from you intended. My admin page (using active_admin) allows a preview of an outgoing email newsletter, in a show view:

    <iframe src="/admin/mail_view/newsletter" />

Sorry I didnt add a test for this change, I didn't know how or where to add it. Cheers.