Prevent MessageBus from hanging forever in the second call to Dispose

It really should be [safe to dispose twice](http://msdn.microsoft.com/en-us/library/system.idisposable.dispose.aspx). [Autofac was even forced to work around this issue](https://code.google.com/p/autofac/wiki/SignalRIntegrationReleaseNotes).

#1088