Fix behavior of Comparable methods
when <=> return nil
Add Enumerable#{flat_map,collect_concat}
Merge pull request #1901 from ksss/compar-other

Fix behavior of Comparable methods when <=> return nil
Merge pull request #1902 from suzukaze/add-enum-flat_map

Add Enumerable#{flat_map, collect_concat}
flat_map to support multiple values; ref #1902