Do not throw exception when calling DirectoryInfoWrapper.Parent for the root directory (#430)

Fixes #429

Author: wexman

System.IO.Abstractions.TestingHelpers.Tests/DirectoryInfoTests.cs

@@ -0,0 +1,22 @@
+using NUnit.Framework;
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace System.IO.Abstractions.TestingHelpers.Tests
+{
+    [TestFixture]
+    public class DirectoryInfoTests
+    {
+        [Test]
+        public void Parent_ForRootDirectory_ShouldReturnNull()
+        {
+            var wrapperFilesystem = new FileSystem();
+
+            var current = wrapperFilesystem.Directory.GetCurrentDirectory();
+            var root = wrapperFilesystem.DirectoryInfo.FromDirectoryName(current).Root;
+            var rootsParent = root.Parent;
+            Assert.IsNull(rootsParent);
+        }
+    }
+}

System.IO.Abstractions/DirectoryInfoWrapper.cs

@@ -227,7 +227,17 @@ public override void SetAccessControl(DirectorySecurity directorySecurity)
 
         public override DirectoryInfoBase Parent
         {
-            get { return new DirectoryInfoWrapper(FileSystem, instance.Parent); }
+            get
+            {
+                if (instance.Parent == null)
+                {
+                    return null;
+                }
+                else
+                {
+                    return new DirectoryInfoWrapper(FileSystem, instance.Parent);
+                }
+            }
         }
 
         public override DirectoryInfoBase Root