fails:Enumerable#to_h converts empty enumerable to empty hash
fails:Enumerable#to_h converts yielded [key, value] pairs to a hash
fails:Enumerable#to_h uses the last value of a duplicated key
fails:Enumerable#to_h calls #to_ary on contents
fails:Enumerable#to_h forwards arguments to #each
fails:Enumerable#to_h raises TypeError if an element is not an array
fails:Enumerable#to_h raises ArgumentError if an element is not a [key, value] pair
