Update SkipList. Fixed Issues #137, #138, #139, #140

[Gutsonok]

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

• An issues were first created before opening this pull request
• The new code follows the contribution guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests to ensure that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes