bug: route's label search is return 400

[idbeta]

Bug report

Describe the bug

the lable of route is a:1
when search on route, is return 400.

Request URL: http://127.0.0.1:9000/apisix/admin/routes?label=a%3A1%2C&page=1&page_size=20
Request Method: GET
Status Code: 400 Bad Request
Response: {"code":10000,"message":"malformed label: \"a:1,\"","data":null,"request_id":"23661bdf-dbd4-47f1-9a7c-d0c5783a4cbe"}

How to Reproduce

[starsz]

@juzhiyuan. If only one label, it shouldn't contain ",".

Like this:
http://127.0.0.1:9000/apisix/admin/routes?label=a:1&page=1&page_size=20

And this:
http://127.0.0.1:9000/apisix/admin/routes?label=a:1,b:1&page=1&page_size=20

cc @LiteSun

[LiteSun]

OK, I'll fix it.