join - composeable CPM support



````javascript
function upcase (dependantKey) {
  return Ember.computed(dependantKey, function () {
    return this.get(dependantKey).toUpperCase();
  });
}

var Obj = Ember.Object.extend({
  firstName: 'Jean-Luc',
  lastName:  'Picard',
  formattedName: join('firstName', upcase('lastName'), ' ') // 'Jean-Luc PICARD'
});
````
This is too clever for me too. A blank string is a bug. Either we ignore it or we raise an assertion.

The only reason for that line is to ignore the separator in `join`. I would just remove the separator from the `mainArguments` before passing it to this function.