$add is not compliant with dates

[javiercbk]

From mongo documentation:

"Adds numbers together or adds numbers and a date. If one of the arguments is a date, $add treats the other arguments as milliseconds to add to the date." documentation link.

The current implementation just sums values, this change would make it work as intended

/**
   * Computes the sum of an array of numbers.
   *
   * @param obj
   * @param expr
   * @returns {Object}
   */
  $add (obj, expr) {
    var args = computeValue(obj, expr)
    var date = args.find(function(argument) {
      return argument instanceof Date;
    })
    if (date) {
      args = args.map(function(argument) {
        if (argument instanceof Date) {
          return argument.getTime()
        }
        return argument
      });
    }
    var reduced = reduce(args, function (acc, num) {
      return acc + num
    }, 0);
    if (date) {
      return new Date(reduced);
    }
    return reduced;
  },