TimedSerializer.loads() raises an error when return_timestamp=True

[dnicolodi]

Here is the traceback:

>>> s = itsdangerous.TimedSerializer('a')
>>> token = s.dumps('hello')
>>> s.loads(token)
u'hello'
>>> s.loads(token, return_timestamp=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/private/tmp/dateutils/lib/python2.6/site-packages/itsdangerous.py", line 275, in loads
    return self.load_payload(base64d)
  File "/private/tmp/dateutils/lib/python2.6/site-packages/itsdangerous.py", line 220, in load_payload
    return self.serializer.loads(payload)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/json/__init__.py", line 307, in loads
    return _default_decoder.decode(s)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/json/decoder.py", line 319, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
TypeError: expected string or buffer