Fix duplicating entries in known_hosts when using proxy command

Fixes #107

OpenSSH does not store ip address when using proxy command like:

server.example.com ssh-rsa AAAAA.......

net-ssh was storing it like:

server.example.com,<no hostip for proxy command> ssh-rsa AAAA.....
Merge pull request #176 from mikz/fix-duplicated-known-hosts-with-proxy

Fix duplicating entries in known_hosts when using proxy command