Update SkipList. Fixed Issues #137, #138, #139, #140 #147
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Description
Fixed #137 . This bug occurs when _getNextLevel() method returned 0.
Fixed #138 and #139 . This bug occurs because of skip list was сircular.
Fixed #140. This bug occurs comparing method was called for null object.
I add implementaion for public T this[T item]. I think this should be work like a standard Dictionary type.
I divided a unit test for SkipList on some tests. And added additinal tests.
Checklist