Avoid warnings like this :
```
2020/12/28 20:37:35 [warn] 3368#3368: *1 [lua] _G write guard:12: __newindex(): writing a global Lua variable ('headers') which may lead to race conditions between concurrent requests, so prefer the use of 'local' variables#012stack traceback:#012#011/usr/local/lib/lua/crowdsec/CrowdSec.lua:85: in function 'allowIp'#012#011access_by_lua(main-lua.conf:166):105: in main chunk, client: 1.3.3.7, server: some.server.com, request: "GET /randomfile1 HTTP/2.0", host: "some.server.com"
```
