MockFileInfo.Append should create a new file if missing (#570)

Fixes #567
Co-authored-by: Florian Greinacher <fgreinacher@users.noreply.github.com>

Author: pianomanjh

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

@@ -230,6 +230,26 @@ public void MockFileInfo_AppendText_ShouldAddTextToFileInMemoryFileSystem()
             Assert.AreEqual($"Demo text contentThis should be at the end{Environment.NewLine}", newcontents);
         }
 
+        [Test]
+        public void MockFileInfo_AppendText_ShouldCreateFileIfMissing()
+        {
+            var fileSystem = new MockFileSystem();
+            var targetFile = XFS.Path(@"c:\a.txt");
+            var fileInfo = new MockFileInfo(fileSystem, targetFile);
+
+            using (var file = fileInfo.AppendText())
+                file.WriteLine("This should be the contents");
+
+            string newcontents;
+            using (var newfile = fileInfo.OpenText())
+            {
+                newcontents = newfile.ReadToEnd();
+            }
+
+            Assert.That(fileSystem.File.Exists(targetFile), Is.True);
+            Assert.AreEqual($"This should be the contents{Environment.NewLine}", newcontents);
+        }
+
         [Test]
         public void MockFileInfo_OpenWrite_ShouldAddDataToFileInMemoryFileSystem()
         {

System.IO.Abstractions.TestingHelpers/MockFileInfo.cs

@@ -152,7 +152,6 @@ public override string Name
 
         public override StreamWriter AppendText()
         {
-            if (MockFileData == null) throw CommonExceptions.FileNotFound(path);
             return new StreamWriter(new MockFileStream(mockFileSystem, FullName, MockFileStream.StreamType.APPEND));
         }