Test failure with QuickCheck 2.14.3

```
  Summation
    Kahan
      t_sum_shifted:                                         FAIL
        *** Failed! Falsified (after 62 tests and 16 shrinks):
        [2.0,6.0,0.0,0.1,0.0]
        Use --quickcheck-replay=740601 to reproduce.
        Use -p '/Kahan.t_sum_shifted/' to rerun this test only.
```

It passes with QuickCheck 2.14.2 or older versions, with everything else unchanged.