Add failing test, can record array promise be resolved multiple times?

I realized that the `DS.Model` `promise` can be resolved multiple times, however `DS.RecordArray` does not work in the same way.

I PR a failing test to show the behaviour. 

I don't still know which is the expected behaviour, because i got a little confused with some tests i made with `Em.DeferredMixin`. The following test didn't also work:

```js
asyncTest("Can re-resolve a promise", function() {
  var value = { value: true };

  var promise = Ember.Deferred.promise(function(deferred) {
    setTimeout(function() {
      Ember.run(function() { deferred.resolve(value); });
    });
  });

  promise.then(function(resolveValue) {
    equal(resolveValue, value, "The resolved value should be correct");

    promise.then(function(resolveValue) {
      start();
    });

});
```

Maybe, i didn't understand well something obvious.