Allow HelperProxy to handle respond_to?

There are cases where, in my decorator, I'd like to make sure the context responds to a method before calling it (eg `#current_user`).

```ruby
class MyDecorator < Draper::Decorator
  def something_with_current_user
    h.current_user if h.respond_to?(:current_user)
  end
end
```

Adding `#respond_to_missing?` allows me to ask if the view context has the method I'm looking for.