$merge deletes properties that are undefined

[ngryman]

Hey Justin,

This code gives me an unexpected output:

const initialState = {
  paymentInfo: {
    cardNumber: '',
    cardExpiry: ''
  }
}

const paymentInfo = {
  cardNumber: undefined,
  cardExpiry: '12/22'
}

const nextState = update(['paymentInfo', $merge(paymentInfo)], state);

Current value of nextState:

{
  paymentInfo: {
    cardExpiry: '12/22'
  }
}

Expected value of nextState:

{
  paymentInfo: {
    cardNumber: '',
    cardExpiry: '12/22'
  }
}

I think that somehow the fact that that cardNumber is undefined makes qim discarding it entirely when cloning the object. I didn't dive deeper though...