Make KeysTest work better

C# does not allow modifications for collections while they are iterated, so get copy of keys and run with that.

There is now special case for top level holdering ("" as key), otherwise transformer is also executed for it (yes, this is ugly and needs fixing).

Author: mcraiha

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);
         }