Allow Skin.AddAttachment to overwrite safely

Allow Skin.AddAttachment to overwrite an existing attachment as intended.
( For more info, see the Remarks section of: http://msdn.microsoft.com/en-us/library/k7z0zy8k%28v=vs.110%29.aspx )

spine-csharp/src/Skin.cs

@@ -45,7 +45,7 @@ namespace Spine {
 
 		public void AddAttachment (int slotIndex, String name, Attachment attachment) {
 			if (attachment == null) throw new ArgumentNullException("attachment cannot be null.");
-			attachments.Add(new KeyValuePair<int, String>(slotIndex, name), attachment);
+			attachments[new KeyValuePair<int, String>(slotIndex, name)] = attachment;
 		}
 
 		/// <returns>May be null.</returns>