Math.abs(-0) should be 0

[deian]

Hi all, Great stuff. I think there is a bug in Math.abs:

javascript-semantics/stdlib/08.math.js

Lines 5 to 11 in d5aca30

	Math.abs = function (x) {
	x = @ToNumber(x);
	if (x < 0)
	return -x;
	else
	return x;
	};

According to the spec, abs of -0 should be 0 not 0.