different behavior of instance_eval

In CRuby, I can get the receiver through block argument, but in mruby, I can't.
Is it mruby's limitation?

``` shell-session
$ cat tmp.rb
class Foo
  def initialize(&block)
    self.instance_eval(&block)
  end
end

Foo.new{ |arg| p arg }

$ ruby tmp.rb
#<Foo:0x007f97ca038798>
$ ./bin/mruby tmp.rb
nil
```
