Issue1478

I've found this error trying to using WeakKeyDictionary as default value
of the defaultdict. 
It's strange because the WeakKeyDictionary is callable as callable
function said it, but not callable for defaultdict.


Jython 2.5.0 (Release_2_5_0:6476, Jun 16 2009, 13:33:26) 
[Java HotSpot(TM) 64-Bit Server VM (Sun Microsystems Inc.)] on java1.6.0_16
>>> import weakref
>>> weakref.WeakKeyDictionary
<class weakref.WeakKeyDictionary at 0x1>
>>> callable(weakref.WeakKeyDictionary)
True
>>> from collections import defaultdict
>>> d = defaultdict(weakref.WeakKeyDictionary) 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: first argument must be callable
>>> d = defaultdict(int)
>>>

fixed in r6816