JSON null string argument should be left as None not casted to 'None'

Given the following request body:

```json
{
  "stuff": null
}
```

And the following parser:

```python
parser = RequestParser()
parser.add_argument('stuff', type=str, location='json')
```

The parser will convert `stuff` to `'None'` since `str(None) == 'None'` instead of leaving it as `None`.

The `convert` method of the `Argument` class should first check if the value is `None` and the expected type is a subclass of `basestring` and leave the value as `None`.