ObjectHelper.isEqual fails if compared object has cyclic links

[matsbryntse]

Such objects cannot be stringified:

case typeA === 'object' && a.constructor.prototype === b.constructor.prototype:
                    return useIsDeeply ? this.isDeeplyEqual(a, b, useIsDeeply) : JSON.stringify(a) === JSON.stringify(b);

Uncaught TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Object'
    |     property 'items' -> object with constructor 'Array'
    |     index 0 -> object with constructor 'Object'
    --- property 'parent' closes the circle
    at JSON.stringify (<anonymous>)
    at Function.isEqual (ObjectHelper.js:85)
    at isEqual (Model.js:26)
    at Model.js:1008
    at Array.forEach (<anonymous>)
    at TestEx.inSet (Model.js:992)
    at TestEx.set (Model.js:941)