rtspsrc: always flush connection in stop
rtspsrc: always flush connection in stop

Use the flush argument in the send_cmd method to force a flush of the
connection. We only want to flush the connection when stopping so that
we can stop and join the task.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=684924

knuesel/gst-plugins-good,knuesel/gst-plugins-good,knuesel/gst-plugins-good,knuesel/gst-plugins-good