In order to get the "replace facts" command to work, I had to do
```
payload = {'name'=>'example', 'values'=> {'virtual' => 'stuff'}}
client.command('replace facts', payload.to_json, 1)
```

An example of how to use `command` would be helpful for disambiguating the right format, but since other parts of this client (such as Client options) use Ruby's `=>` hash syntax, for consistency it seems like the `payload` argument should also have that syntax, and here we should call `payload.to_json`.