Gst.Object: fix creation of GValues for property setter in custom code
Gst.Object: fix creation of GValues for property setter in custom code

The GValue needs to hold the type of the value, it's not related to
"this" or "property".

gstreamer-sharp/gstreamer-sharp,freedesktop-unofficial-mirror/gstreamer__gstreamer-sharp,GStreamer/gstreamer-sharp,freedesktop-unofficial-mirror/gstreamer__gstreamer-sharp,GStreamer/gstreamer-sharp,freedesktop-unofficial-mirror/gstreamer__gstreamer-sharp,gstreamer-sharp/gstreamer-sharp,GStreamer/gstreamer-sharp,gstreamer-sharp/gstreamer-sharp