Angular Karma post has unexpected JSON

I have the following karma test. But I get the error "EXPECTED: {"data":{"form_field_17":"None Required","title":"Mr","firstName":"John","lastName":"Doe","email":"[email protected]","mobile":"1114269874","registration":"123","form_field_15":"Test","pref_date":"2016-08-30T02:13:00.000Z"},"formId":4} GOT: {"data":"{"form_field_17":"None Required","title":"Mr","firstName":"John","lastName":"Doe","email":"[email protected]","mobile":"1114269874","registration":"123","form_field_15":"Test","pref_date":"30/08/2016 12:13 PM"}","formId":4}

Why does my JSON object have backslashes and my pref_date has changed format?

describe('when submitting a form', function () {
        it('should be successful', function () {
            var d = {
                form_field_17: "None Required",
                title: "Mr",
                firstName: "John",
                lastName: "Doe",
                email: "[email protected]",
                mobile: "1114269874",
                registration: "123",
                form_field_15: "Test",
                pref_date: "2016-08-30T02:13:00.000Z"
            };

            $httpBackend.expect('POST', '/DynamicForm/SubmitForm/', { data: d, formId: 4})
                .respond(200, "[{ success : 'true', data : 'true' }]");

            DynamicFormFactory.SubmitForm(d, 4)
              .then(function (data) {
                  expect(data.success).toBeTruthy();
              });

            $httpBackend.flush();

            $httpBackend.verifyNoOutstandingExpectation();
            $httpBackend.verifyNoOutstandingRequest();
        });
    });