Fix issue 3565

    1) for |a| or |b| = oo: handle the case of geometic series
       with eval_sum_hyper()
    2) in doit(): work arround the case when eval_sum returns
       partially evaluated sum with Piecewise
Remove ancient, commented out, code from Sum.doit()

    This fragment is in place long time
    ago (commit 157dd374e728e37eebef729584cd9e94fad02bc8,
    issue 577) for no reason.