Issue description

最近在做网关测试，路由里边设置了百度的首页网址，因为百度有做防爬限流所以会报502异常，这个理论上是正常的，问题是运行了一个晚上服务器一直报：
021/08/31 08:41:12 [error] 49#49: *17163909 lua entry thread aborted: runtime error: stack overflow
stack traceback:
coroutine 0:
[C]: in function 'nkeys'
/usr/local/apigw/apisix/core/table.lua:108: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'
..., client: 10.244.4.108, server: _, request: "GET /key/auth?times=46217 HTTP/1.1"

请求这个是什么原因，有什么影响呢，能否修复这个bug？是不是内存没有正常释放呢

附上路由配置：
http://baidu.com

Environment

• apisix version (cmd: apisix version):
• OS (cmd: uname -a):
• OpenResty / Nginx version (cmd: nginx -V or openresty -V):
• etcd version, if have (cmd: run curl http://127.0.0.1:9090/v1/server_info to get the info from server-info API):
• apisix-dashboard version, if have:
• luarocks version, if the issue is about installation (cmd: luarocks --version):

Steps to reproduce

如上描述

Actual result

/usr/local/apigw/apisix/core/table.lua:111: in function '_deepcopy'

Error log

如上描述

Expected result

直接返回502，不应该报这个递归的copy异常