update() method creates fields even fallback is false

The following test shows new field 'prop.id' is created when updating field 'p' even that the form's fallback option is false:

it('no fallback on separated mode - 2', () => {
  const fields = ['prop']
  const values = {
      prop: {
          id: 'id 1',
          desc: 'description 1'
      }
  }
  const form = new Form({fields, values}, {name: 'Form without fallback', options: {fallback: false}})
  expect(form.$('prop').value).to.deep.equal({
      id: 'id 1',
      desc: 'description 1'
  })
  form.$('prop').update({
      id: 'id 2',
      desc: 'description 2'
  })
  expect(form.select('prop.id', undefined, false)).to.be.undefined // fails. field prop.id is created
  expect(form.$('prop').value).to.deep.equal({  // fails, field prop's value is not changed
      id: 'id 2',
      desc: 'description 2'
  })
})

foxhound87 / mobx-react-form

update() method creates fields even fallback is false #602