[BUGFIX beta] Allow unbound helpers to add attributes.

In prior versions of Ember you could render attributes via:

```
<div {{someHelper}}></div>
```

This brings that back.

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@mixonic / @mmun - Definitely looking for feedback on this one.

Fixes https://github.com/emberjs/ember.js/issues/9961.
Yes, agreed.  I mentioned in the  main comments that I can make this whole path fire a deprecation warning.  This would make it clear that we are going to remove this in 2.0...