Add copy constructor to `TagHelperAttribute`.

- Added test to validate copy constructor does its job.

#492

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperAttribute.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using Microsoft.Framework.Internal;
 
 namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
 {
@@ -19,6 +20,17 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         {
         }
 
+        /// <summary>
+        /// Instantiates a new instance of <see cref="TagHelperAttribute" /> with values provided by the given
+        /// <paramref name="attribute"/>.
+        /// </summary>
+        /// <param name="attribute">A <see cref="IReadOnlyTagHelperAttribute"/> whose values should be copied.</param>
+        public TagHelperAttribute([NotNull] IReadOnlyTagHelperAttribute attribute)
+            : this (attribute?.Name, attribute?.Value)
+        {
+            Minimized = attribute.Minimized;
+        }
+
         /// <summary>
         /// Instantiates a new instance of <see cref="TagHelperAttribute"/> with the specified <paramref name="name"/>
         /// and <paramref name="value"/>.

test/Microsoft.AspNet.Razor.Runtime.Test/TagHelpers/TagHelperAttributeTest.cs

@@ -0,0 +1,36 @@
+// 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 Xunit;
+
+namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
+{
+    public class TagHelperAttributeTest
+    {
+        public static TheoryData CopyConstructorData
+        {
+            get
+            {
+                return new TheoryData<IReadOnlyTagHelperAttribute>
+                {
+                    new TagHelperAttribute("hello", "world") { Minimized = false },
+                    new TagHelperAttribute("checked", value: null) { Minimized = true },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(CopyConstructorData))]
+        public void CopyConstructorCopiesValuesAsExpected(IReadOnlyTagHelperAttribute readOnlyTagHelperAttribute)
+        {
+            // Act
+            var tagHelperAttribute = new TagHelperAttribute(readOnlyTagHelperAttribute);
+
+            // Assert
+            Assert.Equal(
+                readOnlyTagHelperAttribute,
+                tagHelperAttribute,
+                CaseSensitiveTagHelperAttributeComparer.Default);
+        }
+    }
+}