form_fields() can't handle some documents

I recently encountered a site grab couldn't handle. Here is the code to reproduce that issue:

```python
from grab import Grab

g = Grab()
g.setup_document('''
<form>
    <input id="aa" type="radio" name="xxx" value="1111" disabled="disabled" />
    <input id="bb" type="radio" name="xxx" value="2222" checked="checked" />
</form>
''')
print g.form_fields()
```