set(values) {
    if (!this.isValid) return this;

    const mixed = { ...this.values, ...normalizeObject(values, Duration.normalizeUnit) };
    return clone(this, { values: mixed });
  }
"Set" the values of specified units. Return a newly-constructed Duration.
@param {Object} values - a mapping of units to numbers
@example dur.set({ years: 2017 })
@example dur.set({ hours: 8, minutes: 30 })
@return {Duration}