Change `HtmlString` to inherit from `HtmlEncodedString`

- part of aspnet/HtmlAbstractions#5 fix
- also extend existing special cases to more general `HtmlEncodedString`

src/Microsoft.AspNet.Mvc.Razor/TagHelpers/UrlResolutionTagHelper.cs

@@ -176,7 +176,7 @@ namespace Microsoft.AspNet.Mvc.Razor.TagHelpers
                         var htmlContent = attribute.Value as IHtmlContent;
                         if (htmlContent != null)
                         {
-                            var htmlString = htmlContent as HtmlString;
+                            var htmlString = htmlContent as HtmlEncodedString;
                             if (htmlString != null)
                             {
                                 // No need for a StringWriter in this case.

src/Microsoft.AspNet.Mvc.TagHelpers/TagHelperContentExtensions.cs

@@ -4,6 +4,7 @@
 using System;
 using System.IO;
 using System.Text.Encodings.Web;
+using Microsoft.AspNet.Html;
 using Microsoft.AspNet.Mvc.Razor;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Razor.TagHelpers;
@@ -48,7 +49,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             }
 
             string stringValue;
-            var htmlString = value as HtmlString;
+            var htmlString = value as HtmlEncodedString;
             if (htmlString != null)
             {
                 // No need for a StringWriter in this case.

src/Microsoft.AspNet.Mvc.ViewFeatures/Rendering/HtmlString.cs

@@ -1,59 +1,25 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
-using System;
-using System.IO;
-using System.Text.Encodings.Web;
 using Microsoft.AspNet.Html;
 
 namespace Microsoft.AspNet.Mvc.Rendering
 {
-    /// <summary>
+    /// <inheritdoc />
-    /// String content which knows how to write itself.
+    public class HtmlString : HtmlEncodedString
-    /// </summary>
-    public class HtmlString : IHtmlContent
     {
-        private readonly string _input;
-
         /// <summary>
         /// Returns an <see cref="HtmlString"/> with empty content.
         /// </summary>
         public static readonly HtmlString Empty = new HtmlString(string.Empty);
 
-        /// <summary>
-        /// Returns an <see cref="HtmlString"/> containing <see cref="Environment.NewLine"/>.
-        /// </summary>
-        public static readonly HtmlString NewLine = new HtmlString(Environment.NewLine);
-
         /// <summary>
         /// Creates a new instance of <see cref="HtmlString"/>.
         /// </summary>
-        /// <param name="input"><c>string</c> to initialize <see cref="HtmlString"/>.</param>
+        /// <param name="input">The HTML encoded value.</param>
         public HtmlString(string input)
+            : base(input)
         {
-            _input = input;
-        }
-
-        /// <inheritdoc />
-        public void WriteTo(TextWriter writer, HtmlEncoder encoder)
-        {
-            if (writer == null)
-            {
-                throw new ArgumentNullException(nameof(writer));
-            }
-
-            if (encoder == null)
-            {
-                throw new ArgumentNullException(nameof(encoder));
-            }
-
-            writer.Write(_input);
-        }
-
-        /// <inheritdoc />
-        public override string ToString()
-        {
-            return _input;
         }
     }
 }

test/Microsoft.AspNet.Mvc.ViewFeatures.Test/Rendering/HtmlHelperTest.cs

@@ -125,7 +125,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
             {
                 return new TheoryData<string, string>
                 {
-                    { null, null },
+                    { null, string.Empty },
                     { string.Empty, string.Empty },
                     { "<\">", "<\">" },
                     { "<br />", "<br />" },