rest_qos: fix error of additional a qos rule with "priority"

    Currently, rest_qos is impossible to set the "priority" of when to set the QoS rule to switch.
    This patch make it possible to set the "priority" in the following curl command.

    e.g.)
      curl -X POST -d '{"priority":"5",
                        "match": {"nw_dst": "10.0.0.1", "nw_proto": "UDP", "tp_dst": "5002"},
                        "actions":{"queue": "1"}}' http://localhost:8080/qos/rules/0000000000000001

Reported-by: Jace Liang <b436412@gmail.com>
Signed-off-by: Minoru TAKAHASHI <takahashi.minoru7@gmail.com>