Fix find_by_id and find method returning nil unless .all called before

Fix find_by_id and find method returning nil unless .all called beforehand for ActiveYaml or ActiveFile

This problem is related to this issue, https://github.com/zilkey/active_hash/pull/52, however it's not necessarily a fix.  Fundamentally the way ActiveFile inherits from ActiveHash is a little wrong as there is no way to inject data beforehand.