Windows: public interfaces receives unicode, should transform to utf8 bytes

[Natalia Bidart]

Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\twisted\internet\defer.py", line 1020, in _inlineCallbacks
    result = g.send(result)
  File "E:\u1\controlpanel\improve-ui\ubuntuone\controlpanel\gui\qt\addfolder.py", line 66, in on_clicked
    is_valid = yield self.backend.validate_path_for_folder(folder)
  File "E:\u1\controlpanel\improve-ui\ubuntuone\controlpanel\logger.py", line 78, in inner
    res = f(*args, **kwargs)
  File "C:\Python27\lib\site-packages\twisted\internet\defer.py", line 1141, in unwindGenerator
    return _inlineCallbacks(None, f(*args, **kwargs), Deferred())
--- <exception caught here> ---
  File "C:\Python27\lib\site-packages\twisted\internet\defer.py", line 1020, in _inlineCallbacks
    result = g.send(result)
  File "E:\u1\controlpanel\improve-ui\ubuntuone\controlpanel\backend.py", line 659, in validate_path_for_folder
    if not folder_path.startswith(user_home) or is_link(folder_path):
  File "E:\u1\client\fix-tests\ubuntuone\platform\windows\os_helper.py", line 331, in inner
    args[i] = transformer(args[i])
  File "E:\u1\client\fix-tests\ubuntuone\platform\windows\os_helper.py", line 181, in get_windows_valid_path
    assert_syncdaemon_path(path)
  File "E:\u1\client\fix-tests\ubuntuone\platform\windows\os_helper.py", line 133, in assert_syncdaemon_path
    assert isinstance(path, str), 'Path %r should be a bytes sequence.' % path
exceptions.AssertionError: Path u'C:\\Users\\Test\\Music\\udftest\\' should be a bytes sequence.