Login weirdness

[kousu]

We need to adjust the prepare() method to handle all the exceptional cases. In particular, there's a difference between an empty password, None for a password, and a wrong password -- even when accessing public datasets:

>>> list(osfclient.OSF(username='', password=None).project('fg8jy').storage().files)[0].name
'README.md'
>>> list(osfclient.OSF(username='', password='').project('fg8jy').storage().files)[0].name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/api.py", line 24, in project
    return Project(self._json(self._get(url), 200), self.session)
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/models/core.py", line 23, in _get
    return self.session.get(url, *args, **kwargs)
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/models/session.py", line 45, in get
    raise UnauthorizedException()
osfclient.exceptions.UnauthorizedException
>>> list(osfclient.OSF(username='', password='afdsfs').project('fg8jy').storage().files)[0].name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/api.py", line 24, in project
    return Project(self._json(self._get(url), 200), self.session)
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/models/core.py", line 60, in _json
    status_code))
RuntimeError: Response has status code 502 not (200,)
>>> list(osfclient.OSF(username='sdfiljsdofj', password='afdsfs').project('fg8jy').storage().files)[0].name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/api.py", line 24, in project
    return Project(self._json(self._get(url), 200), self.session)
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/models/core.py", line 23, in _get
    return self.session.get(url, *args, **kwargs)
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/models/session.py", line 45, in get
    raise UnauthorizedException()
osfclient.exceptions.UnauthorizedException
>>> list(osfclient.OSF(username='sdfiljsdofj', password='').project('fg8jy').storage().files)[0].name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/api.py", line 24, in project
    return Project(self._json(self._get(url), 200), self.session)
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/models/core.py", line 23, in _get
    return self.session.get(url, *args, **kwargs)
  File "/home/kousu/.local/lib/python3.6/site-packages/osfclient/models/session.py", line 45, in get
    raise UnauthorizedException()
osfclient.exceptions.UnauthorizedException