ImageWidget render() bypasses FileInput.render()

ImageWidget has the following line:
```python
        input_html = super(forms.FileInput, self).render(name, value, attrs)
```

I don't understand why it's bypassing the FileInput render() method, which would otherwise set value=None so it doesn't appear in the rendered input field.  This seems like a bug, and in fact it breaks testing with WebTest for me, but I don't know if there was a reason you did it this way.