Mark StoreCreator's preloadedState argument as optional

[srittau]

When wrapping createStore() using typescript, the typescript definition prevents an optional preloadedState argument:

function myCreateStore<S>(reducer: Reducer<S>, preloadedState?: Redux.DeepPartial<S>) {
    return createStore(reducer, preloadedState);
}

This will complain that type undefined is not assignable to DeepPartial<S>. Marking the preloadedState argument as optional fixes this problem.

[timdorr]

Interesting that was overlooked, since StoreEnhancerStoreCreator already has an optional preloadedState. Funky.

Thanks for the one-character fix!