gstobject: fix double string escaping in gst_object_default_deep_notify()

gstobject: fix double string escaping in gst_object_default_deep_notify()

Make output of gst-launch -v readable again.

last-message = "event\ \ \ \*\*\*\*\*\*\*\ \(fakesink0:sink\)\ E\ \(type:\ tag\ \(20510\)\,\ GstTagList-stream\,\ taglist\=\(taglist\)\"taglist\\\,\\\ video-codec\\\=\\\(string\\\)H264\\\,\\\
 minimum-bitrate\\\=\\\(uint\\\)636611\\\,\\\ bitrate\\\=\\\(uint\\\)980729\\\,\\\ maximum-bitrate\\\=\\\(uint\\\)1116707\\\;\"\;\)\ 0x15bc760"

vs.

last-message = event   ******* (fakesink0:sink) E (type: tag (20510), GstTagList-stream, taglist=(taglist)"taglist\,\ video-codec\=\(string\)H264\,\ minimum-bitrate\=\(uint\)856039\,\ bitrate
\=\(uint\)1019748\,\ maximum-bitrate\=\(uint\)1116707\;";) 0x11149e0