fix inspect method

test code

```ruby
class Hoge
  def initialize
    @hoge = "hoge"
  end
end

puts Hoge.new.inspect
```

 * before
```
$ bin/mruby test.rb
#<Hoge:0x8ef0808 @hoge="hoge"
```

 * after
```
$ bin/mruby test.rb
#<Hoge:0x82973c8 @hoge="hoge">
```