Make tests stronger

[SuperMegaGiperProger]

The tests of fromJSON function uses Rectangle function like this:

it.optional('fromJSON should return the object of specified type from JSON representation', function () {
        var MockType = function(a,b,c) {
            this.a = a;
            this.b = b;
            this.c = c;
        };

        [
            {
                proto: tasks.Rectangle.prototype,
                json: '{ "width":10, "height":20 }',
                expected: new tasks.Rectangle(10, 20)
            },{
                proto: MockType.prototype,
                json: '{ "a":10, "b":20, "c":30 }',
                expected: new MockType(10,20,30)
            }
        ].forEach(data => {
            var actual = tasks.fromJSON(data.proto, data.json);
            assert.deepEqual(
                actual,
                data.expected,
                'fromJson method shoud restore all properties from json'
            );
            assert.equal(
                actual.__proto__,
                data.expected.__proto__,
                'fromJson method shoud restore type from prototype argument'
            );
        });
    });

These tests expect that Rectangle prototype has the method getArea, but Rectangle can be implemented in other way and Rectangle tests allow to do it. For example, tests allow to implement Rectangle like this:

function Rectangle(width, height) {
    return {width: width, height: height, getArea: () => width * height}
}

In this example Rectangle prototype doesn't have getArea method. So, this case should be checked in tests.

Why is it important? If you implement Rectangle in wrong way, you will get message that fromJSON function works incorrect, even if it is implemented in right way.