Fix initial value of all store

[aralroca]

With this change this useStore(initialValue) (or with getStore / withStore) is valid. However, is doing an Object.assign with the parameter + the current store, this way only add the missing store variables and is not removing the existint ones.

const { getStore } = createStore({ cart: { price: 0 }})
getStore({ username: 'Aral' })
// Now the store is { username: 'Aral', cart: { price: 0 } }
// Now the initial store is { username: 'Aral', cart: { price: 0 } }

The difference with setStore is that setStore is replacing all the store
without touching the initialStore:

const { getStore } = createStore({ cart: { price: 0 }})
const [,setStore] = getStore()
setStore({ username: 'Aral' })
// Now the store is { username: 'Aral' }
// The initial store is the same that before: { cart: { price: 0 } }

Proposal for future releases: RenamegetStore to another name because do more things than get... For example myStore...

[aralroca]

I need to reduce 2 bytes 😅

[danielart]

go for that 2 bytes! 💪

[aralroca]

I close this PR because with this other PR there is no more initial value, so the same can be done directly with setStore(store => ({ ...store, newProperty: 'blabla' })). And it's already normal that useStore({ someValue: 2 }) does not work, because as it already exists the store does not change it. So it only works for properties (useStore.someValue(2)). What I will do is explain it better in the documentation.