-[NSArray reduce:withBlock:] implementation is incorrect

When initial value is set to ```nil``` and the array consists of single item, this item is returned and reduce block is never evaluated.

There seems to be an issue at https://github.com/supermarin/ObjectiveSugar/blob/master/Classes/NSArray%2BObjectiveSugar.m#L178

I would rather write it as
```
accumulator = block(accumulator, object);
```

Does this make sense?