Compare servers by user, hostname and port

To check if servers match compare them by
user, hostname and port.

When servers match is checked just by hostname and port
there is no way to access to the same server with different users
to execute special actions allowed for specific users.

There are workarounds to do it. But I think it should be done in standard way
through matching/comparing servers by 'user@hostname:port'.

I have found previous merged pull request with similar problem (about port):
https://github.com/capistrano/capistrano/pull/801
and solved my issue in similar way.