Allow plugin registration with empty prefix (e.g. prefix coming from config)

This patch allows handling of empty plugin prefixes, e.g. when the prefix is passed through a config variable:

```javascript
server.pack.register(
    'plugin_name',
    { route: { prefix: config.plugin_name.prefix || '' } },
    function (err) { /* … */ }
);
```
Remove `only`. No idea why you put it there...