Optionally throw error for unsupported arguments

Here's my suggested implementation for issue https://github.com/flask-restful/flask-restful/issues/355

I'm open to suggestions from the web-savvier if there's a more appropriate response than BadRequest.
Looks like this style is not supported in `unittest` under python2.6. This should work instead:

```python
self.assertRaises(exceptions.BadRequest, parser.parse_args, req, strict=True)
```