Issue description

set twice route in the same uri，the second response should not include “preNode”
curl http://127.0.0.1:9080/apisix/admin/routes/1 -H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -d '
...
{"action":"set","node":{...},"prevNode":{...}}

Environment

• apisix version (cmd: apisix version): 1.5
• OS: centos7