fix error and double disconnected actions on stopSignalRHub action

[vulzee]

Fixes issue #26

I propose changing this._errorSubject.next(error); to if (error) { this._errorSubject.next(error); } due to fact this._connection.onclose emits undefined in case of gracefully stopped connection and instance of Error while the error occurred.
This eliminates throw of @ngrx/signalr/stopHub action on stopSignalRHub(hub).

To fix double @ngrx/signalr/disconnected I propose removing this._stateSubject.next(disconnected); because the this._stateSubject.next would be called in this._connection.onclose after the connection stops.

[Odonno]

Looks good to me.