Do not warn if replaceReducer causes reducer shape to change

[Kaivosukeltaja]

Attempts to solve issue #1636.
replaceReducer calls the new reducer with empty objects as parameter to get the desired state shape, then filters out the keys in the current state that don't exist in the new one.
Added new test to verify functionality.
My first PR to Redux, please be gentle :)

[quinnnned]

I don't believe this will work for unusually deep or unusually shallow state trees.

1. Deep: This will only fix the warning for a top-layer combineReducers. A deeper state tree with multiple layers of combineReducers would still produce the warning.

2. Shallow: Consider the following reducer:

export default (state = {}, action) => {
    switch (action.type) {
        case "ADD_KEY_VALUE_PAIR":
            return {
                ...state,
                [action.key]: action.value
            }
        default:
            return state
    }
}

This state tree is a simple collection of key-value pairs. The proposed solution would wipe the state back to an empty object

[timdorr]

I appreciate the PR, but I think #2673 is the way to go here. Thanks anyways!