bug1019528 - use of cachecontrol

https://bugzilla.mozilla.org/show_bug.cgi?id=1019528
Here you are not testing with the cache, because of the global CACHE_SESSION not being updated above. You can see it because you are mocking requests.get, but you should mock CacheControl instead.
I am not sure this unit test is useful because the code of url_links is already tested.
I think it may be better testing that get_http_session returns a CacheControl when it is set (an requests otherwise), and that set_http_session really set a session also.