Fix very subtile bug in `multipart.py`
Fix very subtile bug in `multipart.py`

When adding a directory from a file descriptor (the default for Python3 on
Linux) the top-level added directory would end up being `<dirname>/` rather
then just `<dirname>` which would cause the directory to be sent twice to
the daemon which would then normalize the paths to the same name but treat
them as separate entities hence returning an extra empty directory result
for the first name.

alexander255/py-ipfs-api,ipfs/py-ipfs-api,alexander255/py-ipfs-api,ipfs/py-ipfs-api