Remove (as opposed to setting to null) the current Span from the Http…

…RequestMessage properties on request stop (#53)

src/OpenTracing.Contrib.NetCore/CoreFx/HttpHandlerDiagnostics.cs

@@ -112,7 +112,7 @@ protected override void OnNext(string eventName, object arg)
 
                             span.Finish();
 
-                            request.Properties[PropertiesKey] = null;
+                            request.Properties.Remove(PropertiesKey);
                         }
                     }
                     break;