beforeSave() is not called on session destruction
In my case I am setting the cookie path in the beforeSave() handler. 
However, when I destroy the session setting
```
this.session = null;
```
The handler is not called, resulting in the creation of a new empty cookie with path = `/`and my old cookie still living in the custom path.

What is the best way of solving this? 
- a separate hook for `beforeClear()`
- calling the existing `beforeSave()` also when the value is null

A workaround is to set the session to an empty object to force a save. If the workaround is preferable the docs should probably mention it.