[#2418] Fix WebUI error when adding non-ascii torrent

json.dumps attempts to decode (utf8) the 'path' entry which had a
different encoding. The solution is to ensure the 'path' entry is
utf8 encoded and remove the unneeded 'path.utf-8' entry.

As self.__m_metadata["info"]["files"] is updated the later code
checking and decoding the 'path' entry can be removed.