fails:Enumerator#peek_values returns the next element in self
fails:Enumerator#peek_values does not advance the position of the current element
fails:Enumerator#peek_values can be called repeatedly without advancing the position of the current element
fails:Enumerator#peek_values works in concert with #rewind
fails:Enumerator#peek_values returns an array with only nil if yield is called with nil
fails:Enumerator#peek_values returns an empty array if yield is called without arguments
fails:Enumerator#peek_values raises StopIteration if called on a finished enumerator
