Allow initial state extend

[onedesert]

Allow also to extend the initial state, otherwise you are force to create the full initial state each time you want a particular change on it.

[TristanWright]

I get an initialState from my reducer and const newState = Object.assign({}, initialState) then modify newState as needed. Some deepCopy function could achieve the same, better too because Object.assign only copies top level.

[dmitry-zaets]

It is implemented with redux-things/redux-actions-assertions-js#3
So we can use it.
But it still needs to be documented.