Change return value of Enumberable#each_cons

More compatibility to CRuby.

mrbgems/mruby-enum-ext/mrblib/enum.rb

@@ -124,6 +124,7 @@ module Enumerable
       ary << val.__svalue
       block.call(ary.dup) if ary.size == n
     end
+    nil
   end
 
   ##

mrbgems/mruby-enum-ext/test/enum.rb

@@ -25,8 +25,9 @@ end
 
 assert("Enumerable#each_cons") do
   a = []
-  (1..5).each_cons(3){|e| a << e}
+  b = (1..5).each_cons(3){|e| a << e}
   assert_equal [[1, 2, 3], [2, 3, 4], [3, 4, 5]], a
+  assert_equal nil, b
 end
 
 assert("Enumerable#each_slice") do