'assigned' option defaults to functools.WRAPPER_ASSIGNMENTS

[benoitbryon]

As of version 0.5.1, assigned option of Decorum class defaults to ('__doc__', '__name__').
What about using functools.WRAPPER_ASSIGNMENTS:

• would be consistent with a feature of Python's standard library
• functools.WRAPPER_ASSIGNMENTS has not the same value in Python 2 and Python 3. I suppose it reflects meaningful changes, i.e. decorum may follow this evolution.