fix typing: initialState can be optional (#29)

* Update index.d.ts

initialState can be NULL also.

* Update index.d.ts

Overload for when no initalState is provided.

Author: tuuling

index.d.ts

@@ -4,7 +4,7 @@ type Action = {
 
 type Reducer<S> = (state: S, action: Action) => S;
 
-export default function reduceReducer<S>(
-  initialState: S,
-  ...reducers: Array<Reducer<S>>
-): Reducer<S>;
+function reduceReducers<S>(initialState: S | null, ...reducers: Reducer<S>[]): Reducer<S>;
+function reduceReducers<S>(...reducers: Reducer<S>[]): Reducer<S>;
+
+export default reduceReducer;