gdbus: Work around a D-Bus bug with path_namespace='/' match rules

D-Bus versions < 1.6.18 (i.e. all current versions) have a bug with the
path_namespace='/' match rule key. It should conceptually match everything,
but actually matches nothing. This results in no property change (or other)
signals being forwarded to the D-Bus client.

The work-around implemented in GDBusObjectManagerClient is to remove the
path_namespace match key if its value is ‘/’.

For the upstream D-Bus bug, see:
https://bugs.freedesktop.org/show_bug.cgi?id=70799

https://bugzilla.gnome.org/show_bug.cgi?id=710726