title: RecyclerView-selection with multiple viewTypes
body: <p>I using the Paging3 and RecyclerView-selection libraries. I used the Paging API to insert separators and headers, all works correctly, but when a header or separators are clicked the selection is cleared completely. I don't want something to happen when headers or separators are clicked.</p>
  <p>When my ItemDetailsLookup try to cast as HeaderViewHolder or SeparatorViewHolder, null is returned and the selection is cleared.</p>
  <pre><code>class MyItemDetailsLookup(
          private val recyclerView: RecyclerView
      ) : ItemDetailsLookup&lt;Long&gt;() {
          
          override fun getItemDetails(event: MotionEvent): ItemDetails&lt;Long&gt;? {
              val view = recyclerView.findChildViewUnder(event.x, event.y)
              if (view != null) {
                  return (recyclerView.getChildViewHolder(view) as? TransactionViewHolder)?.getItemDetails()
              }
              return null
  
          }
      }
  </code></pre>