in `Mongo::Server::Connection#authenticate!`
there is a logical test that read as :

```
if options[:user] || options[:auth_mech]
...
end
```

That seems incorrect in case the .yml file has no :user entry but a :auth_mech is provided.

In that case, the Authenticate! could be called with a Nil for user and then code calling
@user.xxx will fails miserably.

It seems to me, that the mongoid.yml file sample, should point out, that in case of 'no security', the
`auth_mech` should not by set.

Or the logic test in code, should test as :

```
def authenticate!
if options[:user] || options[:auth_mech]
user = Auth::User.new(Options::Redacted.new(:auth_mech => default_mechanism).merge(options))
@server.handle_auth_failure! do
Auth.get(user).login(self)
end
end
end
```