sort_index does not work with ascending=
ascending=[False] is treated as ascending=[True]

> > > t=pd.DataFrame({'A':['x','y','z']})
> > > t.sort(['A'], ascending=[True])
> > >    A
> > > 0  x
> > > 1  y
> > > 2  z
> > > t.sort(['A'], ascending=[False])
> > >    A
> > > 0  x
> > > 1  y
> > > 2  z
