fix: Fix error "assignment to entry in nil map" of Files.Upload() when config.HTTPOptions is nil (#235)

Current code is nil-checking `config.HTTPOptions.Headers`, not `httpOptions`.

Also, `config` is not used afterward, so setting `config.HTTPOptions.Headers` to an empty `http.Header` is not needed.

Co-authored-by: happy-qiao <[email protected]>

Author: meinside