Wrong return type of combineReducers

Hello,

I am using `redux-immutable` with `TypeScript` and found this issue.

The `combineReducer` (provided by `redux-immutable`) returns a plain object instead of `Immutable.Collection`.

Such typing force me to do `state.sidebar` instead of `state.get('sidebar')` in `mapStateToProps`.

```javascript
const mapStateToProps = (state: StateType) => ({
sidebar: state.get('sidebar'), // <- this is correct example of usage
})
```

Start at the combineReducer entry point and inspect its TypeScript return type. Reproduce the reported mapStateToProps usage, then verify that the combined state is typed as an Immutable collection and that state.get('sidebar') type-checks correctly.

Written by the indexing model from the issue text.