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.
`req` isn't `None`, but some of the tests, like `test_namespace_configurability`, throw `RuntimeError('working outside of request context')` because `req` is 

    (Pdb) req
    <LocalProxy unbound>
    (Pdb) type(req) 
    <class 'werkzeug.local.LocalProxy'>

Checking `if req` catches that.  That's just the solution I blundered to, though, I'd be happy to replace it with something better.