getInitialState prevents composing reducers

[rstacruz]

For bigger apps, it's usually a good practice to create multiple reducers and combine them. Here's an example using compose (or reduce-reducers):

function shops (state, action) {
  ...
}
function users (state, action) {
  ...
}
createStore(compose([ shops, users ])

In these cases, having an initialState is redundant because there's no need to make initial state on each sub-reducer.

You could've instead done:

function addDefault (state, action) {
  if (typeof state === 'undefined') return {} /* initial state */
  return state
}

createStore(compose([ addDefault, shops, users ])

The fix is rather easy... just make getInitialState optional.