Keep part.byteCount undefined in chunked encoding

This sets `part.byteCount` to `undefiend` instead of a negative number when there is not `Content-Length` available, which would be when the `Transfer-Encoding` is chunked and the part does not have a `Content-Length` header. Prior to this, `part.byteCount` would be a negative number, which was useless.

Fixes #55