Fix QDeclarativeBluetoothSocket stringData property

Task-number: QTBUG-31729

Change-Id: Idd820e57b72c2293c999bdc0a578d1e8f8e5471b
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>