Adding url without "http://" is considered not a URL

Even with cleaning the data provided, it's still saved as the original input

```
def clean(self):
    cleaned_data = self.cleaned_data
    url = cleaned_data.get('url')

    # if url is not empty and doesn't start with 'http://'

    if url and not url.startswith('http://'):
        url = 'http://' + url
        cleaned_data['url'] = url

        return cleaned_data
```
