Fix code example for Message#resent_sender=

The code example for Message#resent_sender= contains the wrong method name
Merge pull request #520 from RKushnir/patch-1

Fix code example for Message#resent_sender=
Docs: mail.from/to/bcc/etc return emails only. Closes #529.
Merge branch 'master' of git://github.com/mikel/mail