request help: use this lib in nginx stream subsystem

Hi folks, thank you for open sourcing this lib.

I tried using this lib in the Nginx stream subsystem, and it works well, but I found that the judgment here always comes up when running the test case:

https://github.com/cloudflare/lua-resty-logger-socket/blob/15cc1c256e55b8e68ec9b220b6883c227a763d4e/lib/resty/logger/socket.lua#L31-L39

In the Nginx http subsystem, `ngx.config` looks like this:

```
{
debug = true,
nginx_configure = ,
nginx_version = 1019009,
ngx_lua_version = 10020,
prefix = ,
subsystem = "http"
}
```

and in Nginx stream subsystem,`ngx.config` looks like this:

```
{
debug = true,
nginx_configure = ,
nginx_version = 1019009,
ngx_lua_version = 10,
prefix = ,
subsystem = "stream"
}
```

I would be happy to submit a PR to improve this issue if you agree to be compatible with Nginx streams subsystem here.