[ASCollectionNode] Add scrollableDirections method to ASCollectionViewLayoutInspecting p… (#2381)

* Add scrollableDirections method to ASCollectionViewLayoutInspecting protocol

ASViewController's default method will not always return the correct scrollable
directions in cases where size changes may not have been applied. This allows
us to ignore this issue for a little while longer by allowing the layout
inspector to explicitly vend this.

* Better header, thanks @maicki!

AsyncDisplayKit/Details/ASCollectionViewFlowLayoutInspector.m

@@ -176,6 +176,11 @@ static inline ASSizeRange NodeConstrainedSizeForScrollDirection(ASCollectionView
   return [self layoutHasSupplementaryViewOfKind:kind inSection:section collectionView:collectionView] ? 1 : 0;
 }
 
+- (ASScrollDirection)scrollableDirections
+{
+  return (self.layout.scrollDirection == UICollectionViewScrollDirectionHorizontal) ? ASScrollDirectionHorizontalDirections : ASScrollDirectionVerticalDirections;
+}
+
 #pragma mark - Private helpers
 
 - (CGSize)sizeForSupplementaryViewOfKind:(NSString *)kind inSection:(NSUInteger)section collectionView:(ASCollectionView *)collectionView