fix the property cache

Cache the variant value. We must not cache the QScriptValue, because
it's a reference and the user might change the actual object.

Task-number: QBS-312
Change-Id: I34020e56beed34abe2a6e305f3a52e10f43547d4
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>