Fix 1**z evaluation if z is infinite

closes #8582
On Fri, Dec 12, 2014 at 04:34:09AM -0800, Christopher Smith wrote:
>    Does this mean that (oo+I*oo).is_infinite and (var('a', positive=True,
>    infinite=True) + var('b', positive=True, infinite=True)).is_infinite are
>    both None?

Yes.