overrides: Try hard to make the query writable in the pad query function

overrides: Try hard to make the query writable in the pad query function

Summary:
We know that the bindings will get an extra ref but we know that
it is not actually needed, so we are safe to decrease the refcount
by one in that particular context making sure we give PyGI its
ref back when we are done.

Reviewers: Mathieu_Du

Differential Revision: http://phabricator.freedesktop.org/D41

https://bugzilla.gnome.org/show_bug.cgi?id=746329