[BUGFIX release] attributeBinding on class triggers assertion.

Using `attributeBinding: ['class']` will render your view unable to
handle DOM events (`EventDispatcher` uses `.ember-view` class for
events), `classNameBindings` is the more appropriate place to do this.
Merge pull request #9425 from rwjblue/attribute-binding-class

[BUGFIX release] attributeBinding on class triggers assertion.