useUrlState doesn't delete initialState when state is set to undefined

[Gusis93]

In relation to #890

When trying to delete a state, the state will always go back to its initial state

Steps to reproduce

• Use the hook with an initial state

const [state, setState] = useUrlState({
   someState: 'Initial State
})

• Try to delete said state:

setState({someState: undefined});

Expected:

• someState is not anymore

Actual:

• state is: {someState: 'Initial State'}

[askwuxue]

看文档的描述，其实删除就是还原，不知道是不是这个描述有问题？需要确认，是还原成默认值还是彻底删除。