Fixed attribute change notification when a record is created with initial values.

When creating a record with initial properties, ``context.attributeName`` in ``willSetProperty() (line 187)`` and  ``didSetProperty (line 191)`` in states.js is ``undefined``, so ``_changesToSync`` is populated incorrectly. 

This causes the ``attributeName`` argument of ``DS.Adapter.dirtyRecordsForAttributeChange()`` to be the same for multiple properties.

This has been fixed by using ``context.name`` instead.

I added an integration test for ``Adapter.dirtyRecordsForAttributeChange`` to cover this.

See issue https://github.com/emberjs/data/issues/582