'Assert that expected environment is present when finally called.'

def __call__(self, env, start_response):

    for (k, v) in self.expected_env.items():
        assert (env[k] == v), ('%s    !=    %s' % (env[k], v))
    resp = webob.Response()
    resp.body = six.b('SUCCESS')
    return resp(env, start_response)
