Fixed double escape of & in mail_to helper

* This only manifests itself with mail_to links that have more than one
  query option (subject, body, cc, bcc)
* Escaping & in mail_to is unnecessary, because it is already later
  handled when link_to is called
Merge pull request #1654 from rmm5t/fix-mailto-double-amp

Fixed double escape of & in mail_to helper