[BUGFIX beta] Allow a reduceComputed to have an undefined initialValue.

- Stop `ReduceComputedProperty.prototype.initialValue` from asserting
  that a reduce computed's initialValue must not be undefined.
- Modify `Ember.reduceComputed`'s assertion that initialValue be present
  to check that the object hasOwnProperty instead of isNone.
- Add a test that relies on an undefined initialValue to pass.