Controlled Checkbox array example

[leesei]

When checking "A", "C", the controlled field will return ["a",null,"c"] upon submit. https://react-hook-form.com/docs/usecontroller

Code Sandbox: https://codesandbox.io/s/usecontroller-checkboxes-99ld4?file=/src/App.js

I improved the example using Set: https://codesandbox.io/s/usecontroller-checkboxes-forked-dfxcln?file=/src/App.js

Background issue: https://github.com/react-hook-form/react-hook-form/issues/476

I'm not sure whether the original behavior is intentional. At least it doesn't match the behavior of uncontrolled component. Shall Set be adopted?