Add 'respond_to_missing?' to ActiveDecorator::Helper

When  I want to detect a helper method that is defined in a controller,
I must write like following.

```ruby
module SampleDecorator
  def invoke_helper_method
    ActiveDecorator::ViewContext.current.respond_to?(:helper_method) ? helper_method : nil
  end
end
```

this patch enable to write in this way.

```ruby
module SampleDecorator
  def invoke_helper_method
    respond_to?(:helper_method) ? helper_method : nil
  end
end
```