[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 initialValue is in options instead of isNone.
- Add a test that relies on an undefined initialValue to pass.
- Add a test that an error is thrown when you define a reduceComputed
  without an initialValue property.
Merge pull request #3518 from jamesotron/reduce-computed-undefined

Remove reduceComputed's requirement that initialValue not be undefined.