Harmlessly make ActiveResource more MongoMapper friendly

We're using MongoMapper with ActiveResource. Some of our model attributes begin with an underscore and also have a value that is an array (eg - "'_child' => ['bill', 'sam', 'joe']").

The typical behavior or ActiveResource is to infer a class name from the key when the value is an array. In the example above, the default is to assume that each element of the array is a member of a class called "_child". However, this will raise an exception because constants (like class names) cannot begin with an underscore. Luckily, if the actual elements of the array are simple strings or numbers, than they will be loaded as a simple array (which is what we want).

However, ActiveResource is still a bit too eager in trying to resolve the class name. If it were to simply wait to try to instantiate the inferred class until it was really needed, then everything would work fine. So, in this patch, we simply defer the constantization until it is really needed, if at all.

This simple fix makes everything work.