feat(find): support type guards without casting

Author: rob3c

src/operator/find.ts

@@ -36,8 +36,10 @@ import { Subscriber } from '../Subscriber';
  * @owner Observable
  */
 /* tslint:disable:max-line-length */
-export function find<T>(this: Observable<T>, predicate: (value: T, index: number, source: Observable<T>) => boolean, thisArg?: any): Observable<T>;
-export function find<T, S extends T>(this: Observable<T>, predicate: (value: T, index: number, source: Observable<T>) => value is S, thisArg?: any): Observable<S>;
+export function find<T, S extends T>(this: Observable<T>,
+                                     predicate: ((value: T, index: number, source: Observable<T>) => boolean) |
+                                                ((value: T, index: number, source: Observable<T>) => value is S),
+                                     thisArg?: any): Observable<S>;
 /* tslint:disable:max-line-length */
 export function find<T>(this: Observable<T>, predicate: (value: T, index: number, source: Observable<T>) => boolean,
                         thisArg?: any): Observable<T> {