SC.Observable defines a helper method `getEach` for getting properties on a single object.  This conflicts with SC.Enumerable's `getEach`, which means that you can't call getEach on an SC.Set like you would on an SC.Array implementation.  Instead, when mixing in SC.Enumerable and SC.Observable to SC.Set, we should mix in SC.Observable first, so that SC.Enumerable's version of `getEach` wins.

Includes unit test.