Create previous attributes using deepClone

- This is similar to the change introduced recently for single models, 
but this is for models in relations.

lib/relation.js

@@ -418,7 +418,7 @@ const Relation = RelationBase.extend({
     // its parsed attributes
     related.map((model) => {
       model.attributes = model.parse(model.attributes);
-      model.formatTimestamps()._previousAttributes = _.clone(model.attributes);
+      model.formatTimestamps()._previousAttributes = _.cloneDeep(model.attributes);
       model._reset();
     });