[plugins/router] set `isProcessing` to false on `completeNavigation`

While building my SPA with Durandal I ran into (edge) cases (login/logout with a rebuild of the router) where the router would hang the app as it would not stop navigating anymore as `isProcessing` was never reset.

Force-setting `isProcessing` to `false` on `completeNavigation` (just like on cancel, et. al.) fixed this for me while not affecting any other behavior of the router.