Make KeysTest work better

Json5/Json5.cs

@@ -105,7 +105,8 @@ private static Json5Value Walk(Json5Container holder, string key, Func<Json5Cont
             if (value is Json5Container)
             {
                 Json5Container c = (Json5Container)value;
-                foreach (string k in c.Keys)
+                string[] keys = c.Keys.ToArray();
+                foreach (string k in keys)
                 {
                     Json5Value v = Walk(c, k, transformer);
                     if (v != null)
@@ -115,6 +116,12 @@ private static Json5Value Walk(Json5Container holder, string key, Func<Json5Cont
                 }
             }
 
+            // Special case for holder
+            if (key == "")
+            {
+                return value;
+            }
+
             return transformer(holder, key, value);
         }