Fix parts of workerscript

The worker script creates a special QmlContextWrapper
that doesn't contain many things found in the main thread.
However we still need to be able to cast to it, so it
should be the same class as the regular context wrapper.

Fixes parts of the worker script auto tests.

Change-Id: I3697b2b0080dc4ac967eb447e2efd0f28fbab465
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Fix two more errors in the worker script auto test

Change-Id: Ib1359a985efae18cd4fc8344b81092af57f1a9d4
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>