Correct cache key when finding on has_many collections

Queries like `blog.posts.find_by_id(post.id)` or `blog.posts.find(post.id)` are wrongly concatenating `/first` to the cache key. Updating the resource will try to delete a key which hasn't `/first` concatenating, making your application to keep fetching the old resource from cache.

/cc @grosser @staugaard