fix(geolocation): fix watchPosition return type

closes #741

src/plugins/geolocation.ts

@@ -174,7 +174,7 @@ export class Geolocation {
    * @param {GeolocationOptions} options  The [geolocation options](https://developer.mozilla.org/en-US/docs/Web/API/PositionOptions).
    * @return Returns an Observable that notifies with the [position](https://developer.mozilla.org/en-US/docs/Web/API/Position) of the device, or errors.
    */
-  static watchPosition(options?: GeolocationOptions): Observable<Geoposition | PositionError> {
+  static watchPosition(options?: GeolocationOptions): Observable<Geoposition & PositionError> {
     return new Observable<Geoposition>(
       (observer: any) => {
         let watchId = navigator.geolocation.watchPosition(observer.next.bind(observer), observer.next.bind(observer), options);