Added EncoderExtensions

This API and tests are simply ported from ASP.NET repo.
Initially, when I ported the component, I did not include the extensions in the port.
I want the CoreFx component to be independent from I/O (TextWriter). But there is more work needed,
to make it trully independent, and I am not sure this will ever happen, so I am including the extensions for now.

src/System.Text.Encodings.Web/src/System.Text.Encodings.Web.csproj

@@ -21,6 +21,7 @@
     <Compile Include="System\Text\Encodings\Web\AllowedCharsBitmap.cs" />
     <Compile Include="System\Text\Encodings\Web\CodePointFilter.cs" />
     <Compile Include="System\Text\Encodings\Web\EncoderCommon.cs" />
+    <Compile Include="System\Text\Encodings\Web\EncoderExtensions.cs" />
     <Compile Include="System\Text\Encodings\Web\HexUtil.cs" />
     <Compile Include="System\Text\Encodings\Web\HtmlEncoder.cs" />
     <Compile Include="System\Text\Encodings\Web\ICodePointFilter.cs" />

src/System.Text.Encodings.Web/src/System/Text/Encodings/Web/EncoderExtensions.cs

@@ -0,0 +1,70 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+using System.IO;
+
+namespace System.Text.Encodings.Web
+{
+    /// <summary>
+    /// Helpful extension methods for the encoder classes.
+    /// </summary>
+    public static class EncoderExtensions
+    {
+        /// <summary>
+        /// HTML-encodes a string and writes the result to the supplied output.
+        /// </summary>
+        /// <remarks>
+        /// The encoded value is also safe for inclusion inside an HTML attribute
+        /// as long as the attribute value is surrounded by single or double quotes.
+        /// </remarks>
+        public static void HtmlEncode(this IHtmlEncoder htmlEncoder, string value, TextWriter output)
+        {
+            if (htmlEncoder == null)
+            {
+                throw new ArgumentNullException("htmlEncoder");
+            }
+
+            if (!String.IsNullOrEmpty(value))
+            {
+                htmlEncoder.HtmlEncode(value, 0, value.Length, output);
+            }
+        }
+
+        /// <summary>
+        /// JavaScript-escapes a string and writes the result to the supplied output.
+        /// </summary>
+        public static void JavaScriptStringEncode(this IJavaScriptStringEncoder javaScriptStringEncoder, string value, TextWriter output)
+        {
+            if (javaScriptStringEncoder == null)
+            {
+                throw new ArgumentNullException("javaScriptStringEncoder");
+            }
+
+            if (!String.IsNullOrEmpty(value))
+            {
+                javaScriptStringEncoder.JavaScriptStringEncode(value, 0, value.Length, output);
+            }
+        }
+
+        /// <summary>
+        /// URL-encodes a string and writes the result to the supplied output.
+        /// </summary>
+        /// <remarks>
+        /// The encoded value is safe for use in the segment, query, or
+        /// fragment portion of a URI.
+        /// </remarks>
+        public static void UrlEncode(this IUrlEncoder urlEncoder, string value, TextWriter output)
+        {
+            if (urlEncoder == null)
+            {
+                throw new ArgumentNullException("urlEncoder");
+            }
+
+            if (!String.IsNullOrEmpty(value))
+            {
+                urlEncoder.UrlEncode(value, 0, value.Length, output);
+            }
+        }
+    }
+}

src/System.Text.Encodings.Web/tests/EncoderExtensionsTests.cs

@@ -0,0 +1,72 @@
+// Copyright (c) Microsoft Open Technologies, Inc. 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 Xunit;
+
+namespace System.Text.Encodings.Web
+{
+    public class EncoderExtensionsTests
+    {
+        [Fact]
+        public void HtmlEncode_ParameterChecks()
+        {
+            Assert.Throws<ArgumentNullException>(() => EncoderExtensions.HtmlEncode(null, "Hello!", new StringWriter()));
+        }
+
+        [Fact]
+        public void HtmlEncode_PositiveTestCase()
+        {
+            // Arrange
+            IHtmlEncoder encoder = new HtmlEncoder(UnicodeRanges.All);
+            StringWriter writer = new StringWriter();
+
+            // Act
+            encoder.HtmlEncode("Hello+there!", writer);
+
+            // Assert
+            Assert.Equal("Hello&#x2B;there!", writer.ToString());
+        }
+
+        [Fact]
+        public void JavaScriptStringEncode_ParameterChecks()
+        {
+            Assert.Throws<ArgumentNullException>(() => EncoderExtensions.JavaScriptStringEncode(null, "Hello!", new StringWriter()));
+        }
+
+        [Fact]
+        public void JavaScriptStringEncode_PositiveTestCase()
+        {
+            // Arrange
+            IJavaScriptStringEncoder encoder = new JavaScriptStringEncoder(UnicodeRanges.All);
+            StringWriter writer = new StringWriter();
+
+            // Act
+            encoder.JavaScriptStringEncode("Hello+there!", writer);
+
+            // Assert
+            Assert.Equal(@"Hello\u002Bthere!", writer.ToString());
+        }
+
+        [Fact]
+        public void UrlEncode_ParameterChecks()
+        {
+            Assert.Throws<ArgumentNullException>(() => EncoderExtensions.UrlEncode(null, "Hello!", new StringWriter()));
+        }
+
+        [Fact]
+        public void UrlEncode_PositiveTestCase()
+        {
+            // Arrange
+            IUrlEncoder encoder = new UrlEncoder(UnicodeRanges.All);
+            StringWriter writer = new StringWriter();
+
+            // Act
+            encoder.UrlEncode("Hello+there!", writer);
+
+            // Assert
+            Assert.Equal("Hello%2Bthere!", writer.ToString());
+        }
+    }
+}

src/System.Text.Encodings.Web/tests/System.Text.Encodings.Web.Tests.csproj

@@ -30,6 +30,7 @@
     <Compile Include="CodePointFilterTests.cs" />
     <Compile Include="CommonTestEncoder.cs" />
     <Compile Include="EncoderCommonTests.cs" />
+    <Compile Include="EncoderExtensionsTests.cs" />
     <Compile Include="Extensions.cs" />
     <Compile Include="HtmlEncoderTests.cs" />
     <Compile Include="JavaScriptStringEncoderTests.cs" />