Add license header

Author: josesimoes

nanoFramework.Json.Test/Configuration/SettingsTests.cs

@@ -1,4 +1,9 @@
-using nanoFramework.Json.Configuration;
+//
+// Copyright (c) .NET Foundation and Contributors
+// See LICENSE file in the project root for full license information.
+//
+
+using nanoFramework.Json.Configuration;
 using nanoFramework.Json.Resolvers;
 using nanoFramework.Json.Test.Mocks;
 using nanoFramework.TestFramework;