Change return value of Enumerable#each_slice

More compatibility to CRuby.

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

@@ -154,6 +154,7 @@ module Enumerable
       end
     end
     block.call(ary) unless ary.empty?
+    nil
   end
 
   ##

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

@@ -31,8 +31,9 @@ end
 
 assert("Enumerable#each_slice") do
   a = []
-  (1..10).each_slice(3){|e| a << e}
+  b = (1..10).each_slice(3){|e| a << e}
   assert_equal [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10]], a
+  assert_equal nil, b
 end
 
 assert("Enumerable#group_by") do