Allow addClassOneTime to handle Array values returned from source property

That allows us to apply multiple classes to be added at once.

Use case: You want to style elements in a calendar when they have appointments in the past,future and today in different styles.

```
var CalenderEntry = 
get classNames() {
      classes = []
      classes.push(this.hasAppointments? "appointments" : "noAppointments");
      today = new Date();
      today.setHours(0, 0, 0, 0);
      thisDate = new Date(this.startDate);
      thisDate.setHours(0, 0, 0, 0);
      if (this.startDate < today) {
        classes.push("past");
      } else if (this.startDate > today) {
        classes.push("future");
      } else {
        classes.push("today");
      }
      return classes;
    }

// In HTML
<div class="entry" data-win-bind="className:classNames addClassOneTime">
```