overwrite id must be string

wrong

```
copy( false, overwrite: {
      id: 2
}
)

```
[ember-data-copyable] Error: Assertion Failed: You cannot index an internalModel by an empty id'
    at Object.assert (index.js:163)
    at InternalModelMap.set (-private.js:6662)



this works

```
copy( false, overwrite: {
      id: '2'
}
)

```