Add test that the user hash lookup request is actually cached

[dbu]

seems we only fix this in https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/pull/152/files#r19523122

[ddeboer]

Yikes! Good one.

[ddeboer]

Seems this is validated already in https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/blob/master/Tests/Unit/EventListener/UserContextSubscriberTest.php#L97, so can we close this?

[stof]

Yeah, the same PR also updated the test, which was previously testing that the request was not cached: https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/pull/152/files#diff-7