Fix "method_complexity" issue in Mbh/Collection/Traits/Sort.php

[lucasdc6]

Function mergeSort has a Cognitive Complexity of 21 (exceeds 5 allowed). Consider refactoring.

https://codeclimate.com/github/MBHFramework/structures/Mbh/Collection/Traits/Sort.php#issue_5a2058b973a730000100001c