Fix ASTextNode truncation string (#1992)

Initial PR to open source our internal ASDK patch list from Google Inc. :)

Author: Yue-Wang-Google

AsyncDisplayKit/ASTextNode.mm

@@ -1278,8 +1278,8 @@ - (NSAttributedString *)_prepareTruncationStringForDrawing:(NSAttributedString *
   NSMutableAttributedString *truncationMutableString = [truncationString mutableCopy];
   // Grab the attributes from the full string
   if (_attributedText.length > 0) {
-    NSAttributedString *originalString = _truncationAttributedText;
-    NSInteger originalStringLength = _truncationAttributedText.length;
+    NSAttributedString *originalString = _attributedText;
+    NSInteger originalStringLength = _attributedText.length;
     // Add any of the original string's attributes to the truncation string,
     // but don't overwrite any of the truncation string's attributes
     NSDictionary *originalStringAttributes = [originalString attributesAtIndex:originalStringLength-1 effectiveRange:NULL];