fix filters init logic

When there are multiple `http` block in conf file, the filter list will be initialized according the last block. 
e.g.
```c
http {
    server {
        listen 80;
        location / {
            proxy_pass http://127.0.0.1:8080;
            header_filter_by_lua_file "hf.lua";
        }
    }
}

http {
    server {
        listen 8080;
        location / {
            return 200 "8080";
        }
    }
}
```

Or maybe NGINX should ensure only one `http` block in conf. 
Please review the code. Thanks.