Mapping bug with blocks with multiple arguments

The issue was that 
```ruby
[:a,:b,:c].each_with_index.to_enum.mapping {|i,j| [i,j] }.to_a
```
returned
```ruby
[[:a, nil], [:b, nil], [:c, nil]]
```
whereas
```ruby
[:a,:b,:c].each_with_index.to_enum.map {|i,j| [i,j] }
```
returned
```ruby
[[:a, 0], [:b, 1], [:c, 2]]
```

I fixed it for mapping for ruby 1.9. The issue might be present for all the other methods on all rubies ...