Fixed issue in memoryfs.makedir where a path containing a forward slash at the end would create a directory with an empty filename

After testing, there is no unicode in six module. six.text_type represents unicode in py2 and str in py3.

Pyftpdlib 1.x uses unicode, the provided path will already be unicode and thus does not require decoding.
Detect if the provided path is unicode or not before attempting decoding. Use six.unicode to preserve compatibility with py3.

r844 modified test_archivefs.py to not run the TestReadArchiveFS tests if libarchve isn't available. This commit "completes" that change by adding the same test-skipping to TestWriteArchiveFS too.