Prevent wsgi from modifying 401 responses.

Question

If I have the following pylons controller:

def challange(self):
    response.status = '401 Unauthorized'
    return 'You are not logged in.'

I would expect to see 'You are not logged in.' But I see :

Unauthorized

This server could not verify that you are authorized to access the document you requested. Either you supplied the wrong credentials (e.g., bad password), or your browser does not understand how to supply the credentials required.

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WSGI Server

I have disabled the pylons error middleware. But the wsig server seems to be modifying the response. How do I stop this?