fails:Enumerator#next_values returns the next element in self
fails:Enumerator#next_values advances the position of the current element
fails:Enumerator#next_values advances the position of the enumerator each time when called multiple times
fails:Enumerator#next_values works in concert with #rewind
fails:Enumerator#next_values returns an array with only nil if yield is called with nil
fails:Enumerator#next_values returns an empty array if yield is called without arguments
fails:Enumerator#next_values raises StopIteration if called on a finished enumerator
