Add copyright header from Mono

Contributes to #24043

Source from Mono: https://github.com/mono/mono/blob/0af1ea70c29a66a59765183b45d6b75fa6c3d8fa/mcs/class/System.Web/System.Web/HttpUtility.cs#L3-L30

Author: karelz

src/System.Web.HttpUtility/src/System/Web/HttpUtility.cs

@@ -2,6 +2,34 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+// Authors:
+//   Patrik Torstensson (Patrik.Torstensson@labs2.com)
+//   Wictor Wilén (decode/encode functions) (wictor@ibizkit.se)
+//   Tim Coleman (tim@timcoleman.com)
+//   Gonzalo Paniagua Javier (gonzalo@ximian.com)
+//
+// Copyright (C) 2005-2010 Novell, Inc (http://www.novell.com)
+//
+// Permission is hereby granted, free of charge, to any person obtaining
+// a copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to
+// permit persons to whom the Software is furnished to do so, subject to
+// the following conditions:
+// 
+// The above copyright notice and this permission notice shall be
+// included in all copies or substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+//
+
 using System.Collections.Specialized;
 using System.Globalization;
 using System.IO;