add autoresponder_from param when turning autoresponder on

The webfaction docs say autoresponder_from is optional
http://docs.webfaction.com/xmlrpc-api/apiref.html#mailboxes
but when an empty string is passed for that param in update_email
the autoresponder functionality does not seem to work.

This uses the email address the user logged in with instead of adding
an email address field.