Possible issue in `MULTIPART_VALUE_COND`

1. https://gitlab.com/os85/httpx/-/blob/master/lib/httpx/transcoder/multipart.rb#L12-14 should to_hash be called before the key? checks?
2. I am not sure if the condition from https://gitlab.com/os85/httpx/-/blob/master/lib/httpx/transcoder/multipart/part.rb#L10 should also be added to MULTIPART_VALUE_COND.