Add non-boxing enumerator for HtmlNodeCollection

Author: lahma

src/HtmlAgilityPack.Shared/HtmlNodeCollection.cs

@@ -168,6 +168,15 @@ public void CopyTo(HtmlNode[] array, int arrayIndex)
             _items.CopyTo(array, arrayIndex);
         }
 
+        /// <summary>
+        /// Get Enumerator
+        /// </summary>
+        /// <returns></returns>
+        public List<HtmlNode>.Enumerator GetEnumerator()
+        {
+            return _items.GetEnumerator();
+        }
+
         /// <summary>
         /// Get Enumerator
         /// </summary>