Add Content-length to response

I'm using ajax for django forms.
Therefore i need to load new captcha with ajax.
example django view:
```python
def process_form(request):
    #some form processing code
    challenge, response = settings.get_challenge()()
    store = CaptchaStore.objects.create(challenge=challenge, response=response)
    key = store.hashkey
    image_url = reverse('captcha-image', kwargs=dict(key=key))
    return HttpResponse(json.dumps( {'key':key, 'url':image_url} ), mimetype="application/json")
```
When i did try this code:
```javascript
//ajax request to get_new_captcha
$('img.captcha').attr('src',url);
```
Firebug log example: http://img42.imageshack.us/img42/5092/93527288.png

Sometime response size returned incorrect. And image not displayed.
With my fix everything work fine.