Use a string for class property keys

ActiveFedora::Base.class.properties are strings, so getting and setting them should use strings for the keys. When setting values, ActiveTriples::Resource won’t save the value, so we call it as a method on the base object directly.

Not sure if this is a good solution, but it seems to fix the issue at #536.


@jcoyne it will set the value, but after `.save` it's not persisted, i.e. `[]`.