[BUGFIX] Undesired Asset ViewHelper instance reuse avoided by clone

This fixes an issue where multiple assets added in the same context (for example inside the same Fluid f:section) would not be included properly.

The cause was an unfortunate mix of Fluid wanting to reuse the ViewHelper instance rather than creating a new one - and VHS wanting to save a reference to the current ViewHelper rather than a clone.

By simultaneously using `clone $this` before finalising each Asset and resetting cached settings in internal properties, instance uniqueness has been achieved.