compute checksum crash when LazyFileContent used

[Maciej Kisielewski]

plainbox crashes when comparing units if at least one of them is using LazyFileContent

See attached file with test for details

[Maciej Kisielewski (kissiel)]

Backtrace from Checkbox-Touch
Traceback (most recent call last):
  File "/home/kissiel/test/checkbox/checkbox-touch/py/checkbox_touch.py", line 171, in wrapper
    result = func(*args, **kwargs)
  File "/home/kissiel/test/checkbox/checkbox-touch/py/checkbox_touch.py", line 362, in start_session
    self.context.add_provider(provider)
  File "/home/kissiel/test/checkbox/checkbox-touch/lib/py/plainbox/impl/session/state.py", line 334, in add_provider
    self.add_unit(unit)
  File "/home/kissiel/test/checkbox/checkbox-touch/lib/py/plainbox/impl/session/state.py", line 350, in add_unit
    if unit in self._unit_list:
  File "/home/kissiel/test/checkbox/checkbox-touch/lib/py/plainbox/impl/unit/unit.py", line 440, in __eq__
    return self.checksum == other.checksum
  File "/home/kissiel/test/checkbox/checkbox-touch/lib/py/plainbox/impl/unit/unit.py", line 717, in checksum
    self._checksum = self._compute_checksum()
  File "/home/kissiel/test/checkbox/checkbox-touch/lib/py/plainbox/impl/unit/unit.py", line 740, in _compute_checksum
    default=default_fn)
  File "/usr/lib/python3.4/json/__init__.py", line 237, in dumps
    **kw).encode(obj)
  File "/usr/lib/python3.4/json/encoder.py", line 192, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python3.4/json/encoder.py", line 250, in iterencode
    return _iterencode(o, 0)
  File "/home/kissiel/test/checkbox/checkbox-touch/lib/py/plainbox/impl/unit/unit.py", line 735, in default_fn
    raise TypeError
TypeError