Use native Date.prototype.toISOString() if it's available

[RA80533]

plist.js/lib/build.js

Line 121 in 5877ec7

next_child.ele('date').txt(ISODateString(new Date(next)));

if (typeof Date.prototype.toISOString === "function") {
    next_child.ele('date').txt(new Date(next).toISOString());
} else {
    next_child.ele('date').txt(toISOString(new Date(next));
}

[mreinstein]

I would accept a PR For this that just uses Date.prototype.toISOString everywhere. We're going to assume much more javascript runtime for plist.js v4