Demonstrate problem where item.save() encounters an error [tests RED]

When item.save() gets a 400 Bad Request response, it chokes parsing the null `data` value without ever calling the save() callback.

test/unit/CollectionItem.js

@@ -10,7 +10,7 @@ describe('CollectionItem', function () {
 	var restHandlersPath = path.normalize(__dirname + '/../../lib/restHandlers');
 
 	var restHandlerMock = {
-		editItem: sinon.spy(),
+		editItem: sinon.stub(),
 		listItems: sinon.spy()
 	};
 
@@ -23,6 +23,7 @@ describe('CollectionItem', function () {
 		var CollectionItem = proxyquire('./../../lib/CollectionItem', stubs);
 
 		item = new CollectionItem('deals', {
+			id: 1,
 			title: 'Deal title'
 		}, 1);
 
@@ -64,4 +65,48 @@ describe('CollectionItem', function () {
 			sinon.assert.calledWith(restHandlerMock.editItem, 2, 'deals/1/products', {id: 2});
 		});
 	});
+
+	describe('item.save()', function() {
+		it('should call restHandler.editItem()', function () {
+			// In the normal case, we can set a field value and call save().  Internally, the restHandler.editItem()
+			// method will be called with our update.
+			item.set('title', 'Other deal title');
+			item.save();
+
+			sinon.assert.calledWith(restHandlerMock.editItem, 1, 'deals', {title: 'Other deal title'});
+		});
+
+		it('should handle error with null data from REST call', function () {
+			// Set a field to a field ID that doesn't exist (maybe the custom field key is bad).
+			item.set('badFieldId', 'Ignored value');
+
+			// When we save the item in this state, the real RestHandler.editItem() is given bad input, so it receives
+			// back an error response, such as a 400 with body:
+			//    {
+			//      "success":false,
+			//      "error":"Bad request",
+			//      "error_info":"Please check developers.pipedrive.com for more information about Pipedrive API.",
+			//      "data":null,
+			//      "additional_data":null
+			//    }
+			//
+			// We want to emulate this behavior to make sure our CollectionItem callback handles it properly.  When a
+			// item.save(callbackFn) call is made, the callbackFn should have the opportunity to handle this error.
+			var restError = new Error('Pipedrive API error:Bad request');
+			restHandlerMock.editItem.callsFake(function(id, kind, changedData, callbackFn) {
+				callbackFn(restError, null, null);
+			});
+
+			// For purposes of this test, we don't need to do anything in our callback, but we MUST ensure that we get
+			// called back.  Without getting the error object passed to it, we can't respond to the rejected input.
+			var saveCallback = sinon.spy();
+			item.save(saveCallback);
+
+			sinon.assert.calledWith(restHandlerMock.editItem, 1, 'deals', {'badFieldId': 'Ignored value'});
+			sinon.assert.calledOnce(saveCallback);
+			sinon.assert.calledWith(saveCallback, restError);
+
+			restHandlerMock.editItem.resetBehavior();
+		});
+	});
 });