If the content_type attribute of a response is set to None in a view, th...

## Description
If the content_type attribute of a response is set to None in a view, the resulting response will not have the 'content_type' attribute. This results in an error in cornice's validator where it assumes 'content_type' will always be an attriubte of the passed in response object. I do not know why Pyramid does this, but it does it. The example below responds to a delete request with a 204 with no content and no content_type (a successful delete).

### Example:
```python

@resource(collection_path='/resource/', path='/resource/{id}')
class ActionView:

  def __init__(self, request):
    self.request = request
   
  def delete(self):
     # do some stuff
    self.request.response.content_type = None
    self.request.response.status_code = 204
    return 
```

When delete is called, cornice errors :

```bash
  File "/xxxxx/eggs/cornice-0.14-py3.3.egg/cornice/validators.py", line 13, in filter_json_xsrf
    if response.content_type in ('application/json', 'text/json'):
nose.proxy.AttributeError: 'AttributeError' object has no attribute 'content_type'
```
### Versions
pyramid: 1.4.3
cornice: 0.14

## Testing Notes
I tried running the tests before creating this pull request, but I had to do a bit of work to get it to work using python 3.3. After changing some files to get buildout to work with Python 3.3, I ran the tests (nosetests inside cornice directory) and got errors:

```bash
  File "/home/gewthen/dev/cornice/cornice/schemas.py", line 79, in validate_colander_schema
    from colander import Invalid, Sequence, drop
ImportError: cannot import name drop
```