[csharp] Fixed an error in `Skin` where `Skin.setAttachment()` did not work as expected on 3.8 branch (See #1485).

spine-csharp/src/Collections/OrderedDictionary.cs

@@ -277,6 +277,7 @@ namespace Spine.Collections
 			set {
 				int index;
 				if (dictionary.TryGetValue(key, out index)) {
+					keys[index] = key;
 					values[index] = value;
 				}
 				else {