Edited Issue: [CORS] Put EnableCors under namespace System.Web.Http [913]

The HttpConfiguration extension EnableCors is under namespace System.Web.Http.Cors in recent build. As a result, to enable cors in WebApiConfig you need to include 'using System.Web.Http.Cors'.

In contrast with EnableSystemDiagnosticsTracing, which is under System.Web.Http, the later enable cleaner user code.