Fix NSAttributeDescription lookups using attributesByName as described in #536

I’ve been seeing a very specific issue where `NSEntityDescription`’s `attributesByName` dictionary uses an unusual string format internally that is preventing standard `objectForKey:` and object subscripting lookups from working. 

In all cases, switching to enumerating through the returned values and using `isEqualToString:` fixes the problem.

I'm open to alternative approaches to this (it's hardly efficient or simple compared the `objectForKey:` alternative), however there should be minimal impact to performance, as most entities have tens of attributes, not hundreds.