Bug fix: Multiplying by purely imaginary numbers preserves zero sign.

- (* +i (make-rectangular x y)) = (make-rectangular (* -1 y) x)
- (* (make-rectangular x y) +i) = (make-rectangular (* -1 y) x)
- (/ (make-rectangular x y) -i) = (make-rectangular (* -1 y) x)
- (* -1 (make-rectangular x y)) = (make-rectangular y (* -1 x))
- (* (make-rectangular x y) -i) = (make-rectangular y (* -1 x))
- (/ (make-rectangular x y) +i) = (make-rectangular y (* -1 x))
