Update docblocks in DataObject class to reflect correct return values

In my use of the ```create()``` method, it doesn't return a boolean but instead returns an instance of ```OpenCloud\Common\Request\Response\Http```. Since the ```update()``` method is an alias of ```create```, the same should apply there.
Merge pull request #201 from primitive-type/patch-2

Update docblocks in DataObject class to reflect correct return values