Cache: check the whole cache key in addition to hashes.

This prevents a potential attack that discloses cached data if an attacker
will be able to craft a hash collision between some cache key the attacker
is allowed to access and another cache key with protected data.

See http://mailman.nginx.org/pipermail/nginx-devel/2015-September/007288.html.

Thanks to Gena Makhomed and Sergey Brester.