ensure that `state` is immutable

[jjwon0]

why? state should never be mutated. causes more errors than it's worth.

we can use immutable.js to help with this, or try to use some of the helpers that react has already implemented in the library. i feel that immutable.js is better, though.

a couple of options:

• redux-immutable, which creates a combineReducers which works with an immutable initialState
• redux-immutable-state-invariant, which is dev-only and spits out errors if you're mutating it

[jjwon0]

Update: I tried using redux-immutable, but it seems to be not in sync with react-router-redux@4.0.0. redux-immutable-state-invariant doesn't seem to be doing its job. I think it'd be easiest to just wait til the documentation updates.