Fix for .net 3.5

The IsSystemEndpoint property is not avaliable is .net 3.5;
The only system endpoint property in .net 3.5 is metadata endpoint, as
the WS-Discovery is for .net 4.0 and above.
Refer:
http://referencesource.microsoft.com/#System.ServiceModel/ServiceModel/System/ServiceModel/Configuration/ServiceEndpointElement.cs#90d18ba452eaf094#references