`getTimezoneOffset` returns `-0` for UTC

[jamesball27]

Steps to reproduce:

import timezoneMock from 'timezone-mock';

timezoneMock.register('UTC');
new Date().getTimezoneOffset(); // returns -0

this causes issues when using jest because toEqual uses Object.is under the hood, so this assertion fails:

timezoneMock.register('UTC');
const offset = new Date().getTimezoneOffset();
expect(offset).toEqual(0); // fails because Object.is(-0, 0) return false

and instead an ugly workaround is required:

expect(offset === 0).toBe(true);

I'd expect getTimezoneOffset to return 0 for UTC instead of -0
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/getTimezoneOffset#negative_values_and_positive_values