bug: create route with plugin, will be error

[idbeta]

Bug report

Describe the bug

create route with plugin, will be error

How to Reproduce

1. create route with route, only enable the plugin but not do any config, so when finish commit the route, it will prompt an error, which is correct
   

{"name":"test","desc":"","uris":["/name"],"methods":["GET","HEAD","POST","PUT","DELETE","OPTIONS","PATCH"],"priority":0,"plugins":{"limit-count":{"disable":false,"policy":"local","rejected_code":503}},"vars":[],"upstream":{"type":"roundrobin","timeout":{"connect":6000,"send":6000,"read":6000},"nodes":[{"host":"127.0.0.1","port":80,"weight":1}]}}

2. but when I go back to route config, and disable the plugin, when finish commit the route again, it also will prompt an error, which is wrong

{"name":"test","desc":"","uris":["/name"],"methods":["GET","HEAD","POST","PUT","DELETE","OPTIONS","PATCH"],"priority":0,"plugins":{"limit-count":{"disable":true,"policy":"local","rejected_code":503}},"vars":[],"upstream":{"type":"roundrobin","timeout":{"connect":6000,"send":6000,"read":6000},"nodes":[{"host":"127.0.0.1","port":80,"weight":1}]}}

[juzhiyuan]

Need a discussion on if we MUST have valid data no matter enable or disable that plugin.

cc @nic-chen @idbeta @membphis