Fix lost reference to hash (data) passed into RESTAdapter#ajax

- function passed into new RSVP expects hash reference for ajax data
- reference to hash arg is lost after promise constructor call
- added a local reference in the closure scope to prevent lossing scope
- unit tests mock response with resolved promise so don't catch this case

See:
https://github.com/emberjs/data/blob/master/packages/ember-data/lib/adapters/rest_adapter.js#L501-L517
https://github.com/emberjs/data/blob/master/packages/ember-data/tests/integration/adapter/rest_adapter_test.js#L156-L169
https://github.com/emberjs/data/blob/master/packages/ember-data/tests/integration/adapter/rest_adapter_test.js#L34-L42