'The `WWW-Authenticate` header in a parsed form.'

@property
def www_authenticate(self):

    def on_update(www_auth):
        if ((not www_auth) and ('www-authenticate' in self.headers)):
            del self.headers['www-authenticate']
        elif www_auth:
            self.headers['WWW-Authenticate'] = www_auth.to_header()
    header = self.headers.get('www-authenticate')
    return parse_www_authenticate_header(header, on_update)
