Call enqueue interceptor for retained messages (fix) (#2212)

* Call the event handler also for retained messages

* Update release notes

* Add Unit Test

Source/MQTTnet.Tests/BaseTestClass.cs

@@ -24,4 +24,9 @@ public abstract class BaseTestClass
     {
         return Task.Delay(TimeSpan.FromSeconds(1));
     }
+
+    protected Task MediumTestDelay()
+    {
+        return Task.Delay(TimeSpan.FromSeconds(0.5));
+    }
 }