Reset state of SinglyLinkedList after accesing a key on HashTable

* HashTable#[] was maintaining current_node state of its list, which
caused the next access to the same hashed_key to return nil
* Added test for it
* Couldn't keep a one liner.. My ruby skillz are kinda fading away :P
Merge pull request #1 from brunops/master

Reset state of SinglyLinkedList after accesing a key on HashTable