Enumrable#each_with_index return Enumerator if non block given

mrbgems/mruby-enumerator/test/enumerator.rb

@@ -438,6 +438,10 @@ assert 'Integral#times' do
   assert_equal [0,1,2], c
 end
 
+assert 'Enumerable#each_with_index' do
+  assert_equal [['a',0],['b',1],['c',2]], ['a','b','c'].each_with_index.to_a
+end
+
 assert 'Enumerable#map' do
   a = [1,2,3]
   b = a.map

mrblib/enum.rb

@@ -108,6 +108,8 @@ module Enumerable
   #
   # ISO 15.3.2.2.5
   def each_with_index(&block)
+    return to_enum :each_with_index unless block_given?
+
     i = 0
     self.each{|*val|
       block.call(val.__svalue, i)