additional information:
`url-http` calls `url-http-find-free-connection` before `(setq url-current-object url)` and `url-http-find-free-connection` calls `url-open-stream` and `url-open-stream` referencing `url-current-object` before `url-http` updates it's value.
when `url-open-stream` referencing `url-current-object`, it's value is `nil` or old `url-current-object`.

so, let `url-current-object` to `url` in `url-http` before calling `url-http-find-free-connection`.