set cell default background color to clearColor. close jessesquires#727

Author: jessesquires

JSQMessagesViewController/Controllers/JSQMessagesViewController.m

@@ -521,6 +521,7 @@ - (UICollectionViewCell *)collectionView:(JSQMessagesCollectionView *)collection
 
     cell.textView.dataDetectorTypes = UIDataDetectorTypeAll;
 
+    cell.backgroundColor = [UIColor clearColor];
     cell.layer.rasterizationScale = [UIScreen mainScreen].scale;
     cell.layer.shouldRasterize = YES;