utils: filter_values() is not GroupableOrderedDict aware

[kaplun]

Take this snippet:

>>> from dojson.utils import GroupableOrderedDict, force_values
>>> a = GroupableOrderedDict([('a', None)])
>>> @filter_values
>>> def foo(b):
...     return b
>>> foo(a)
{'__order__': ('a',)}

Instead the correct result should have been:

>>> foo(a)
{'__order__': ()}