Fix QJSEngine::newQObject ownership behaviour

Fix QJSEngine::newQObject ownership behaviour

Ensure the indestructible flag is set to false for objects wrapped through
QJSEngine::newQObject, to ensure that they get deleted upon gc when they
don't have a parent.

Re-enabled the QJSEngine::garbageCollect and ownership auto-tests that verified
this behaviour.

Change-Id: I181bff0cc44d071d650a2f73494e49cce6ad538e
Reviewed-on: http://codereview.qt-project.org/2398
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>