Allow content=None when stream=True

Right now if `content=None` when `stream=True`, then you get: 

```
  File "/Users/alex/.virtualenvs/media/lib/python2.7/site-packages/httmock.py", line 194, in intercept
    stream=kwargs.get('stream', False))
  File "/Users/alex/.virtualenvs/media/lib/python2.7/site-packages/httmock.py", line 63, in response
    res.raw = BytesIO(content)
TypeError: must be string or buffer, not None
```
