BUG: rolling_apply(..., center=True) should not append NaNs

Closes #8269.
No, it is not hard-coded to return centered results; center affects the value of offset.

> On Sep 16, 2014, at 11:33 PM, stahlous <notifications@github.com> wrote:
> 
> In pandas/stats/moments.py:
> 
> >      return _rolling_moment(arg, window, call_cython, min_periods, freq=freq,
> > -                           center=center, args=args, kwargs=kwargs)
> > +                           center=False, args=args, kwargs=kwargs)
> So, this is hard-coded to always return "centered" results, correct? It looks like center=False should be removed from the function declaration (and the API change noted in the docs).
> 
> Otherwise, this looks OK to me.
> 
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