Infinite loop when trying to convert N/kg to SI

This should yield a quantity with units m/s^2, or if you consider N to be "proper" SI, it should do nothing. Instead, it seems to throw Quantify into an infinite loop.

Repro:
```
(1.N/1.kg).to_si
```

Cancelling the base units also doesn't seem to be able to "separate" the component units of the Newton:
```
(1.N/1.kg).cancel_base_units! :kg
=> 1 N/kg
```