Raise an error when attempting to merge an Array from mergedProperties

In general, we need to be careful requiring `JSON.stringify` (as it is not available and working properly for older ES3 browsers). However, it is EXTREMELY doubtful that any development is actually done using these ES3 browsers and so as long as our production builds do not require `JSON.stringify` (`Ember.assert` is stripped from production builds) I think using it is perfectly fine.