Leave status responsibility to parent class

Hi, this is a small change but that I found valuable.

Django's `HttpResponse` class checks for the `status` param when it's
initialized, if it's `None` it uses the class attribute
`status_code` and thanks to that we can do things like:

```python
class BadRequest(HttpResponse):
    status_code = 400
```

Now, that doesn't work when inheriting from rest-framework's `Response`:

```python
class BadRequest(rest_framework.response.Response):
    status_code = 400  # Doesn't work as expected, it's always ignored
```

Because a default status of `200` is being specified in
`rest_framework.response.Response`. I think is more Django-friendly to
just leave that status default value to `None` and leave the
responsibility of choosing its value to the parent class: `HttpResponse`.