Allow reduceComputed to have an initial value of 0.

This is necessary to get something like the following computed macro working:

```
Ember.computed.sum = function (dependentKey) {
  return Ember.reduceComputed.call(null, dependentKey, {
    initialValue: 0,

    addedItem: function (accumulatedValue, item, changeMeta, instanceMeta) {
      return accumulatedValue + item;
    },

    removedItem: function (accumulatedValue, item, changeMeta, instanceMeta) {
      return accumulatedValue - item;
    }
  });
};
```