Well, I agree in principle that it might be slightly better to make it possible to serialize the `dict_keys` rather than convert to a list. But, `simplejson` is the one raising the `TypeError` not Cornice:

```
  File "/Users/marca/dev/git-repos/cornice/cornice/util.py", line 36, in __call__
    return json.dumps(data, use_decimal=True)
  File "/Users/marca/dev/git-repos/cornice/.tox/py33/lib/python3.3/site-packages/simplejson/__init__.py", line 323, in dumps
    return _default_encoder.encode(obj)
  File "/Users/marca/dev/git-repos/cornice/.tox/py33/lib/python3.3/site-packages/simplejson/encoder.py", line 249, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/Users/marca/dev/git-repos/cornice/.tox/py33/lib/python3.3/site-packages/simplejson/encoder.py", line 325, in iterencode
    return _iterencode(o, 0)
  File "/Users/marca/dev/git-repos/cornice/.tox/py33/lib/python3.3/site-packages/simplejson/encoder.py", line 226, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: dict_keys([1, 2]) is not JSON serializable
```

so since the error is coming from `simplejson` and `dict_keys` is a built-in type in Python 3 (not a Cornice thing), I don't think Cornice should be responsible for making `dict_keys` serializable; perhaps `simplejson` might add that as a feature though.