Edited Issue: Stack overflow if GetControllerMapping called before 1st request [938]

If tracing is enabled and DefaultHttpControllerSelector.GetControllerMapping() is called before the first request has been processed, the MessageHandler pipeline will be corrupted.
The TraceManager.Initialize() code is idempotent for MessageHandlers only after the pipeline has been initialized at the first request. If called before the pipeline has been initialized, it will redundantly wrap MessageHandlerTracer's around other MessageHandlerTracers.
The net result of this is a corrupt pipeline of MessageHandlers that will cause stack overflow at first request.

The TraceManager.Initialize() needs to protect itself when it finds MessageHandlerTracer's already installed in HttpConfiguration.MessageHandlers