[ActiveSupport] [Cache] Inconsistent behavior in namespaced_key

fix for #8614

```bash

>  Rails.cache.send(:namespaced_key, User.first, {})
 => "users/508fefdb54ca455644000003-20121226093034" 
> Rails.cache.send(:namespaced_key, [User.first, User.last], {})
 => "users/508fefdb54ca455644000003-20121226093034/users/50b7832354ca45f043000011-20121129154539" 
> Rails.cache.send(:namespaced_key, [User.first], {})
 => "508fefdb54ca455644000003" 

# yurk, should probably be "users/508fefdb54ca455644000003-20121226093034" instead
```

Also this is inconsistent with the (internally used) ActiveSupport::Cache.expand_cache_key

```bash

> ActiveSupport::Cache.expand_cache_key User.first
 => "users/508fefdb54ca455644000003-20121226093034" 
> ActiveSupport::Cache.expand_cache_key [User.first, User.last]
 => "users/508fefdb54ca455644000003-20121226093034/users/50b7832354ca45f043000011-20121129154539" 
>   ActiveSupport::Cache.expand_cache_key [User.first]
 => "users/508fefdb54ca455644000003-20121226093034" 

```