code: @Override public int getItemCount() {
      int size = items.size();
      if (hasHeader()) {
        size++;
      }
      if (hasFooter()) {
        size++;
      }
      return size;
    }
docs: Returns the items list size if there is no a header configured or the size taking into account
  that if a header or a footer is configured the number of items returned is going to include
  this elements.