Fire observers for array computed changes.

When dealing with chains of arrays, it is not necessary (or correct) to
fire non-array observers when array contents change, but the array itself
does not.

However, for other `ReduceComputedProperty`s, such as                    
`Ember.computed.max`, it is necessary to fire observers in response to   
upstream array changes.                                                  

Thanks @ssured for pointing this out, and for providing tests!