update tag saving

MediaBrowser.Controller/Entities/TagExtensions.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Linq;
+
+namespace MediaBrowser.Controller.Entities
+{
+    public static class TagExtensions
+    {
+        public static void AddTag(this BaseItem item, string name)
+        {
+            if (string.IsNullOrWhiteSpace(name))
+            {
+                throw new ArgumentNullException("name");
+            }
+
+            if (!item.Tags.Contains(name, StringComparer.OrdinalIgnoreCase))
+            {
+                item.Tags.Add(name);
+            }
+        }
+    }
+}