math.rationalize silently ignores imaginary numbers

Currently, `math.rationalize` seems to ignore imaginary numbers:
```js
// math.js version 9.5.1

// complex coefficient a = (2 + complex(3, 4))
let a = new math.OperatorNode("+", "add", [new math.ConstantNode(2), new math.ConstantNode(math.complex(3, 4))]);
console.log(a.evaluate()); // {re: 5, im: 4}

// polynomial a * x
let poly = new math.OperatorNode("*", "multiply", [a, new math.SymbolNode("x")]);

let coefficients = math.rationalize(poly, true).coefficients;
console.log(coefficients); // [0, 5]
```
I would expect `coefficients` to be `[0, 5+4i]` or for `rationalize` to throw an error.