Wrong solution to cubic equation

[m2-farzan]

Hi again

The solution find_roots returns for x^3 + 36 * x^2 + 144 * x + 64 = 0 is wrong. If you run this code on rustnomial 0.3.2 (or older versions):

fn main() {
    let poly = Polynomial::<f64>::new(vec![1.0, 36.0, 144.0, 64.0]);
    let roots = poly.roots();
    match roots {
        rustnomial::Roots::ManyRealRoots(v) => {
            println!("roots() returned `Many Real Roots` with values:");
            for v_ in v {
                println!("{}", v_);
            }
        },
        _ => {
            println!("roots() returned some other type.");
        }
    }
}

it returns:

roots() returned `Many Real Roots` with values:
-31.053752658339498
1.4912025299402378
-6.43744987160074

but in reality the equation has different roots:

-4
-31.492
-0.50807

(See here for exact forms)