`val(date)` modifies passed date instance

[vfonic]

Hi,

I've just experienced this issue:

var time = date.toTimeString().split(' ')[0]; // get hour and minute (there are better ways to do this)
var sched = later.parse.recur().on(time).time();
var dayOfWeek = later.dayOfWeek.val(date); // NOTE this line might mess up your `date` instance

console.log(dayOfWeek); // 3
console.log(date); // { 2017-12-19T20:00:00.000Z dw: 3 }

The date instance got modified. Is this expected behavior? I continued using date below this code point and received errors. I expected that call to val() won't change my date instance.

It's because this line of code: https://github.com/bunkat/later/blob/5d38cc3779ad0ee42d61675c8209c1e47c07ca93/src/constraint/dayofweek.js#L29-L31