title: Issue creating rootReducer with combineReducers
body: <pre><code>import { combineReducers } from 'redux';
  import { reducers } from './reducers';
  import { IAppAction } from './action';
  import { routerReducer } from 'react-router-redux';
  import { IAppState } from './state/app-state';
  
  const appReducer = combineReducers({
      ...reducers,
      router: routerReducer
  });
  
  export const rootReducer = (state: IAppState, action: IAppAction) =&gt; {
      return appReducer(state, action)
  };
  </code></pre>
  
  <p>I am getting the below error</p>
  
  <pre><code>"Argument of type 'IAppState' is not assignable to parameter of type '{ router: RouterState; app: any; }'.
    Type 'IAppState' is missing the following properties from type '{ router: RouterState; app: any; }': router, app"
  </code></pre>
  
  <p>Uploaded sample to <a href="https://github.com/tharaka86/react-template" rel="nofollow noreferrer">github</a></p>
  
  <p>package versions</p>
  
  <pre><code>"react": "^16.8.6",
  "redux": "^4.0.4",
  "react-router-redux": "^4.0.8",
  "typescript": "3.5.3"
  </code></pre>
  
  <p>Exact same code worked on my older project </p>
  
  <pre><code>"react": "^16.8.4",
  "redux": "^3.7.2",
  "react-router-redux": "^4.0.8",
  "typescript": "^3.3.4000"
  </code></pre>