Use addObject when updating record arrays

Currently using pushObject which means creating an object, adding it to the array, then saving the object results in the object being in the array twice.
Add test for RecordArray change to addObject.

Creating a new model, inserting into an existing RecordArray and calling save should not duplicate the model in the RecordArray.
Merge pull request #299 from Connorhd/patch-1

Use addObject when updating record arrays