correctly handle dicts with 2 values in _is_args_kwds

In generator.test_generator._is_args_kwds, the first subcondition
( len(obj) == 2 ) can match dicts with two items, in which case (unless
the keys for the two items happen to be 0 and 1) the second two
subconditions will raise KeyErrors. This is now treated in the same way
as a TypeError.