Summary: It must only be done when the object is commited.
We can do that in constructed though, as the changes will
anyway be commited when the object is added to a composition.
Also update the tests, as we set properties spearately then
check the stop, we can commit the source at its creation without
removing meaning from the tests.
Reviewers: thiblahute
Differential Revision: http://phabricator.freedesktop.org/D84