Add pure ToStringDemystified extension method that does not change exception's state.

test/Ben.Demystifier.Test/ToDemystifiedStringTests.cs

@@ -0,0 +1,48 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Text;
+using System.Threading.Tasks;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Ben.Demystifier.Test
+{
+    public sealed class ToDemystifiedStringTests
+    {
+        private readonly ITestOutputHelper _output;
+
+        public ToDemystifiedStringTests(ITestOutputHelper output)
+        {
+            _output = output;
+        }
+
+        [Fact]
+        public void DemystifyShouldNotAffectTheOriginalStackTrace()
+        {
+            try
+            {
+                SimpleMethodThatThrows().Wait();
+            }
+            catch (Exception e)
+            {
+                var original = e.ToString();
+                var stringDemystified = e.ToStringDemystified();
+
+                _output.WriteLine("Demystified: ");
+                _output.WriteLine(stringDemystified);
+
+                _output.WriteLine("Original: ");
+                var afterDemystified = e.ToString();
+                _output.WriteLine(afterDemystified);
+
+                Assert.Equal(original, afterDemystified);
+            }
+
+            async Task SimpleMethodThatThrows()
+            {
+                throw new InvalidOperationException("message");
+            }
+        }
+    }
+}