[BUGFIX beta] Start on mergedProperties fix for #5182

This provides a partial fix for #5182 (https://github.com/emberjs/ember.js/issues/5182). The reason it's currently a partial fix is due to the fact that Ember.merge does not perform a "deep" merge. I poked around a bit and did not see a supported way to perform a deep merge on objects so I figured I'd submit at least this as a good starting point. Is there something else I should be using here, other than "Ember.merge"?

can this also check `mergedProperties.length` in a manner similar to how `concatenatedProperties` does above? We really only want to do that `indexOf` check if there is something to check against.