Add Enumerable#minmax
Support multiple values in Enumerable#minmax
Hash#each_{key,value} support return Enumerator
if non block given
Merge pull request #1916 from ksss/hash-enumerator

Hash#each_{key,value} support return Enumerator if non block given
Merge pull request #1915 from suzukaze/add-enum.minmax

Add Enumerable#minmax