setting the attribute via `attr` does not update the prop. Changing prop updates the `attr` value if `attr` did not set another value.

given `<input type="radio">`
```javascript
$('input').prop('checked')
  false

$('input').attr('checked')
  false

$('input').prop('checked', true)
  [ <input type=​"radio">​ ]

$('input').prop('checked')
  true

$('input').attr('checked')
  true

$('input').attr('checked', 'checked')
  [<input type=​"radio" checked=​"checked">​]

$('input').prop('checked')
  true

$('input').prop('checked', false)
  [<input type=​"radio" checked=​"checked">​]

$('input').prop('checked')
  false

$('input').attr('checked')
  "checked"
```