new_contents: from pyramid_authsanity.util import add_vary_callback
  
  class TestAddVaryCallback(object):
      def _makeOne(self, *varies):
          return add_vary_callback(varies)
  
      def test_add_single_vary(self):
          cb = self._makeOne('cookie')
          response = DummyResponse()
          cb(None, response)
  
          assert len(response.vary) == 1
          assert 'cookie' in response.vary
  
      def test_add_multiple_vary(self):
          cb = self._makeOne('cookie', 'authorization')
          response = DummyResponse()
          cb(None, response)
  
          assert len(response.vary) == 2
          assert 'cookie' in response.vary
          assert 'authorization' in response.vary
  
      def test_add_multiple_existing(self):
          cb = self._makeOne('cookie')
          response = DummyResponse()
          response.vary = ['cookie']
          cb(None, response)
  
          assert len(response.vary) == 1
          assert 'cookie' in response.vary
  
  
  class DummyResponse(object):
      vary = None
  
subject: Add tests to excercise the vary callback utility