title: React-hook-form doesn't set isDirty to false when back to initial state
body: <p>In the official <a href="https://codesandbox.io/s/react-hook-form-v6-controller-ts-forked-qgx79" rel="noreferrer">exemple for controlled inputs</a> , if you modify the value of an input and then change it back to its initial value, <code>isDirty</code> will be set to true but won't be set back to false and <code>dirtyField</code> will contain the touched field.</p>
  <p>In an older <a href="https://codesandbox.io/s/react-hook-form-v6-formstate-ht098" rel="noreferrer">exemple with uncontrolled inputs</a>, we dont have quite same  behaviour. In fact, if you modify the value of an input and then change it back to its initial value, <code>isDirty</code> will still be falsy but <code>dirtyFields</code> will not contain the touched field.</p>
  <p>Shouldn't <code>isDirty</code> be set back to false when the form is back to its initial state and <code>dirtyFields</code> should be empty?</p>
  <p>Is it the intended behaviour?</p>
  <p>Does the Controllers break the formState?</p>