fix(tap): make next optional (#3073)

closes #2534

Author: felixfbecker

src/operators/tap.ts

@@ -8,7 +8,7 @@ import { noop } from '../util/noop';
 import { isFunction } from '../util/isFunction';
 
 /* tslint:disable:max-line-length */
-export function tap<T>(next: (x: T) => void, error?: (e: any) => void, complete?: () => void): MonoTypeOperatorFunction<T>;
+export function tap<T>(next?: (x: T) => void, error?: (e: any) => void, complete?: () => void): MonoTypeOperatorFunction<T>;
 export function tap<T>(observer: PartialObserver<T>): MonoTypeOperatorFunction<T>;
 /* tslint:enable:max-line-length */