title: ObjC doesn't forward scrollViewDidScroll message correctlly
body: <p>I want to use multicast delegate tech for <code>UIScrollViewDelegate</code>. My general idea is similar with this:
  <a href="https://github.com/tkach/MulticastDelegate/blob/master/MulticastDelegate/MulticastDelegate.m" rel="nofollow noreferrer">https://github.com/tkach/MulticastDelegate/blob/master/MulticastDelegate/MulticastDelegate.m</a></p>
  
  <p>But I couldn't forwarding the <code>scrollViewDidScroll</code> message, I found that I never received this message.
  So are there someone encounter or know the reason of this?
  thx...</p>