Adding test for XmlHelpers.AddAttributeIfNotNullOrEmpty

- We were on the way to introduce an optimization, which would have broken the
  implementation without noticing. Introducing a test to ensure that we'll
  not break things.

Kentor.AuthServices.Tests/Internal/XmlHelpersTests.cs

@@ -76,6 +76,21 @@ public void XmlHelpers_AddAttributeIfNotNullOrEmpty_IgnoresObjectWithEmptyToStri
             e.Attribute("attribute").Should().BeNull();
         }
 
+        [TestMethod]
+        public void XmlHelpers_AddAttributeIfNotNullOrEmtpy_TimeSpanSerializedCorrectly()
+        {
+            // It might be tempting in the implementation to call value.ToString()
+            // instead of passing in the value. That would make types that have
+            // special XML Serialization formats fail. This test ensures that
+            // nobody takes that shortcut without handling the special cases.
+
+            var e = new XElement("xml");
+
+            e.AddAttributeIfNotNullOrEmpty("attribute", new TimeSpan(2, 17, 32));
+
+            e.Attribute("attribute").Should().NotBeNull().And.Subject.Value.Should().Be("PT2H17M32S");
+        }
+
         [TestMethod]
         public void XmlHelpers_GetValueIfNotNull_NullOnNull()
         {
@@ -110,5 +125,6 @@ public void XmlHelpers_GetTrimmedTextIfNotNull_NullOnNull()
 
             e.GetTrimmedTextIfNotNull().Should().BeNull();
         }
+
     }
 }