Fix a broken test

`test_update_cached_response_with_valid_headers` is wrong even though
it currently passes. As far as I can tell it attempts to emulate the
following scenario:
* There is already a cached response for `http://url.com/`
* `CacheController.update_cached_response` is called with a request and
a validation response for the same url
The problem is: it incorrectly sets the second url to `http://foo.com`.
This causes `CacheController.cache.get(cache_url)` to return None.
The test currently passes because `CacheController.serializer.loads`
is mocked and `CacheController.update_cached_response` currently passes
`CacheController.cache.get(cache_url)` to
`CacheController.serializer.loads` unconditionally:
```py
self.serializer.loads(request, self.cache.get(cache_url))
```

This test failed when I tried to refactor the code

tests/test_cache_control.py

@@ -137,7 +137,7 @@ def test_update_cached_response_with_valid_headers(self):
         # skip our in/out processing
         cc.serializer = Mock()
         cc.serializer.loads.return_value = cached_resp
-        cc.cache_url = Mock(return_value="http://foo.com")
+        cc.cache_url = Mock(return_value=self.url)
 
         result = cc.update_cached_response(Mock(), resp)