Fix respond_to?/method_missing inconsistency

Ran into a wierd one today where subclasses of `[DumbDelegator]` would report that they did not respond to methods that were defined directly on the subclass. i.e.

```ruby
class Foo < DumbDelegator
  def bar; end
end 

Foo.new([]).respond_to?(:bar) #=> false
```

As a result I refactored  `[DumbDelegator#respond_to?]` to be more in line with the way `[DumbDelegator#method_missing]` works.