fix: Incorporate Exclude in splitBy signature.

As Moshe used it in ReactiveX/rxjs#5441

Author: cartant

source/operators/splitBy.ts

@@ -9,7 +9,7 @@ import { bucketBy } from "./bucketBy";
 export function splitBy<T, S extends T>(
   predicate: (value: T, index: number) => value is S,
   subjectSelector?: () => Subject<T>
-): OperatorFunction<T, [Observable<S>, Observable<T>]>;
+): OperatorFunction<T, [Observable<S>, Observable<Exclude<T, S>>]>;
 
 export function splitBy<T>(
   predicate: (value: T, index: number) => boolean,
@@ -20,11 +20,9 @@ export function splitBy<T>(
   predicate: (value: T, index: number) => boolean,
   subjectSelector: () => Subject<T> = () => new Subject<T>()
 ): OperatorFunction<T, [Observable<T>, Observable<T>]> {
-  return (
-    bucketBy(
-      2,
-      (value, index) => (predicate(value, index) ? 0 : 1),
-      subjectSelector
-    ) as OperatorFunction<T, [Observable<T>, Observable<T>]>
-  );
+  return bucketBy(
+    2,
+    (value, index) => (predicate(value, index) ? 0 : 1),
+    subjectSelector
+  ) as OperatorFunction<T, [Observable<T>, Observable<T>]>;
 }