You can clone with
# Can be used like:
mail_body = last_email.body.to_s
Per @josevalim, this seems to be the most reasonable way to do it. http://blog.plataformatec.com.br/2013/08/devise-3-1-now-with-more-secure-defaults/#comment-1000254578
Thanks @stevenharman - I've encountered the same problem and your gist really helped. I made some tiny tweaks to the regex and used text_part instead to make it work for me though (see fork).
Thank you! Helped me write my test!