Rehydrate should do a deep merge only on existing keys

On 6.0.0 the rehydrated state is incomplete when the store schema changes. There is a PR to fix this (#114) however there is a scenario it doesn’t cater for, and it was suggested that this scenario should be documented in a new issue.

Scenario

If a key has been removed from the store’s initialState but still included in the saved state in local storage, the rehydrated state will include the deleted key. This is not ideal as it makes it impossible to remove keys from your store schema. This also applies to nested keys.

Example

Store initial state

{
    "key1": {
        "nestedKey1": "value",
        "nestedKey2": "another value",
    },
    "key2": false
}

Saved state in local storage

{
    "key1": {
        "nestedKey1": "update value",
        "nestedKey2": "another value",
        "nestedKey3": "deleted value"
    },
    "key3": true
}

Expected rehydrated state

{
    "key1": {
        "nestedKey1": "updated value",
        "nestedKey2": "another value",
    },
    "key2": false
}

Actual rehydrated state (on #114)

{
    "key1": {
        "nestedKey1": "updated value",
        "nestedKey2": "another value",
        "nestedKey3": "deleted value"
    },
    "key2": false,
    "key3": true
}

If this issue is fixed, it would also be ideal to remove to redundant keys from local storage as well. However this might be complicated especially on nested keys.

[moniuch]

We could now also look into #100 which introduces an option to provide a custom merging function with the config.

[moniuch]

Thank you so much!

@bufke David, what would you say if we provided an issue template with instructions for the users who want to submit a bug ticket, asking them to provide their config and actual/expected state? Just the way @JohnKis did ^^.