Multiplying an unsigned zero produces a signed zero

[Zalathar]

Steps to reproduce:

const Long = require('long');
const x = Long.UZERO.mul(10);

Expected behaviour:

x.unsigned === true

Actual behaviour:

x.unsigned === false

[Zalathar]

This seems to be caused by the check for zero that unconditionally returns ZERO, without consulting self.unsigned:

long.js/src/long.js

Lines 864 to 866 in 9448e61

	LongPrototype.multiply = function multiply(multiplier) {
	if (this.isZero())
	return ZERO;

[dcodeIO]

Yeah, should return this