fs.sshfs 2.0.20 cannot walk a nonunicode filename

fs.sshfs cannot walk the following nonunicode file:
```python
>>> os.listdir('.')
['Ma\udcf1ana']
```
The special character is a Spanish n~, in an unknown, 8 bit encoding.

The error looks like:
```python
File "/home/dstromberg/src/pyfilesystem-tests/lib/python3.6/site-packages/paramiko/sftp_client.py", line 222, in listdir_attr
filename = msg.get_text()
File "/home/dstromberg/src/pyfilesystem-tests/lib/python3.6/site-packages/paramiko/message.py", line 178, in get_text
return u(self.get_string())
File "/home/dstromberg/src/pyfilesystem-tests/lib/python3.6/site-packages/paramiko/py3compat.py", line 143, in u
return s.decode(encoding)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf1 in position 2: invalid continuation byte
```