Added WriteWithAppend

ExcelORM/ExcelORMTests/DynamicWriterTests.cs

@@ -30,6 +30,34 @@ public class DynamicWriterTests
 
         File.Delete(testFile);
     }
+    
+    [Fact]
+    public void WriteWithAppend()
+    {
+        var testFile = Path.GetRandomFileName();
+        testFile = Path.ChangeExtension(testFile, "xlsx");
+
+        using var reader = new ExcelDynamicReader(DifficultFile);
+        var results = reader.Read().ToArray();
+        Assert.NotEmpty(results);
+
+        using var writer = new ExcelDynamicWriter();
+        writer.Write(results);
+        writer.SaveAs(testFile);
+        
+        using var writerAppend = new ExcelDynamicWriter();
+        writer.Write(results, append: true);
+        writer.SaveAs(testFile);
+
+        using var savedReader = new ExcelDynamicReader(testFile);
+        var savedResults = savedReader.Read().ToArray();
+        Assert.NotEmpty(savedResults);
+        Assert.True(results.First().SequenceEqual(savedResults.First()));
+        Assert.True(results.Last().SequenceEqual(savedResults.Last()));
+        Assert.Equal(results.Length * 2, savedResults.Length);
+
+        File.Delete(testFile);
+    }
 
     [Fact]
     public void WriteAll()