Introduced test for verifying exception on circular references

While mapped properties permit circular references due to caching, this isn't possible with circular references when mapping constructor parameters, since the source object won't yet have been created—and, thus, won't yet have been cached. In this case, a `TopicMappingException` is expected. This unit test verifies that is the case. This concludes the testing for mapped constructor parameters (#35).

OnTopic.Tests/TopicMappingServiceTest.cs

@@ -206,6 +206,27 @@ public async Task Map_Constructor_ReturnsNewModel() {
 
     }
 
+    /*==========================================================================================================================
+    | TEST: MAP: CONSTRUCTOR: THROWS EXCEPTION
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>
+    ///   Establishes a <see cref="TopicMappingService"/> and attempts to map a view model with a constructor containing a
+    ///   circular reference, and confirms that a <see cref="TopicMappingException"/> is correctly thrown.
+    /// </summary>
+    [TestMethod]
+    [ExpectedException(typeof(TopicMappingException))]
+    public async Task Map_Constructor_ThrowsException() {
+
+      var topic                 = new Topic("Topic", "Constructed", null, 1);
+      var related               = new Topic("Related", "Constructed", null, 2);
+
+      topic.References.SetValue("TopicReference", related);
+      related.References.SetValue("TopicReference", topic);
+
+      await _mappingService.MapAsync<ConstructedTopicViewModel>(topic).ConfigureAwait(false);
+
+    }
+
     /*==========================================================================================================================
     | TEST: MAP: DISABLED PROPERTY: RETURNS NULL
     \-------------------------------------------------------------------------------------------------------------------------*/