overwrite id must be string

[yaxinr]

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'
}
)