Problem with post request reading in balancer_by_lua_block

[selwynli]

hi
the openresty version of mine is 1.13

I encountered a problem. When I am in the balance_by_lua stage, without ngx.req.read_body(), I can use the get_post_args function to read the parameters of the post; but in the access_by_lua stage, an error will appear; Why is get_post_args() can be used normally in the balance_by_lua stage without ngx.req.read_body()?

tips:
1、It has been confirmed that lua_need_request_body is not turned on
2、ngx.req.read_body() is not read in other stages.

reproduction on the case：

```
upstream hash-forward {
    server 0.0.0.1;  

    balancer_by_lua_block {
        local ngx = require "ngx"
        local cjson = require "cjson.safe"
        local method = ngx.var.request_method

        if method == "POST" then
            local post_args, err = ngx.req.get_post_args()
            if not post_args then
                ngx.log(ngx.ERR, "failed to get post args: ", err)
                return ngx.exit(500)
            end
            local post_str = cjson.encode(post_args)
           
            ngx.log(ngx.ERR, "post params: " .. post_str)
        end

        local balancer = require "ngx.balancer"
        local ok, err = balancer.set_current_peer("127.0.0.1", 8088)
        if not ok then
            ngx.log(ngx.ERR, "failed to set the current peer: ", err)
            return ngx.exit(500)
        end
    }

    keepalive 500;  # connection pool
}
```