feat(filter): support type guards without casting

Author: rob3c

src/operator/filter.ts

@@ -44,9 +44,10 @@ import { TeardownLogic } from '../Subscription';
  * @owner Observable
  */
 /* tslint:disable:max-line-length */
-export function filter<T>(this: Observable<T>, predicate: (value: T, index: number) => boolean, thisArg?: any): Observable<T>;
-export function filter<T, S extends T>(this: Observable<T>, predicate: (value: T, index: number) => value is S, thisArg?: any): Observable<S>;
-/* tslint:disable:max-line-length */
+export function filter<T, S extends T>(this: Observable<T>,
+                                       predicate: ((value: T, index: number) => boolean) |
+                                                  ((value: T, index: number) => value is S),
+                                       thisArg?: any): Observable<S>;
 export function filter<T>(this: Observable<T>, predicate: (value: T, index: number) => boolean,
                           thisArg?: any): Observable<T> {
   return this.lift(new FilterOperator(predicate, thisArg));