fix return value for with_index

mrbgems/mruby-enumerator/mrblib/enumerator.rb

@@ -156,10 +156,10 @@ class Enumerator
     return to_enum :with_index, offset unless block_given?
     raise TypeError, "no implicit conversion of #{offset.class} into Integer" unless offset.respond_to?(:to_int)
 
-    n = offset.to_int
-    each do |i|
-      yield [i,n]
+    n = offset.to_int - 1
+    enumerator_block_call do |i|
       n += 1
+      yield [i,n]
     end
   end
 

mrbgems/mruby-enumerator/test/enumerator.rb

@@ -396,3 +396,12 @@ assert 'modifying existing methods' do
   assert_equal Enumerator, {a:1}.each.class
   assert_equal Enumerator, (1..5).each.class
 end
+
+assert 'Array#map!' do
+  a = [1,2,3]
+  b = a.map!
+  b.with_index do |i, index|
+    [i*i, index*index]
+  end
+  assert_equal [[1,0],[4,1],[9,4]], a
+end