cursor.cursor('path') to immutable.List returns wrong cursor type

``` js
var data = Immutable.fromJS({ a: [1, 2, 3] });
var cursor = Cursor.from(data);
var deepCursor = cursor.cursor('a');
```

deepCursor will be KeyedCursor, not IndexedCursor, due to the way `subCursor()` calls `makeCursor()` and how `makeCursor()` checks for optional arguments.
#297 fixes this issue.
