Not support unicode path

[icersong]

eg: webdav.ls('/软件')

Traceback (most recent call last):
  File "webdavclient.py", line 15, in <module>
    print webdav.ls(u'./软件')
  File "/usr/local/lib/python2.7/dist-packages/easywebdav/client.py", line 176, in ls
    response = self._send('PROPFIND', remote_path, (207, 301), headers=headers)
  File "/usr/local/lib/python2.7/dist-packages/easywebdav/client.py", line 96, in _send
    url = self._get_url(path)
  File "/usr/local/lib/python2.7/dist-packages/easywebdav/client.py", line 104, in _get_url
    path = str(path).strip()
UnicodeEncodeError: 'ascii' codec can't encode characters in position 2-3: ordinal not in range(128)