Make Ember.RenderBuffer#addClass work as expected.

RenderBuffer#addClass currently works in surprising ways, take the following example:

```javascript
var buffer = new Ember.RenderBuffer('div')
buffer.setClasses(['foo', 'bar']);
buffer.addClass('baz'); // I'd say I'm adding 'baz' to the two classes I've set before
buffer.pushOpeningTag();
buffer.pushClosingTag();
```

will produce:

```html
<div class="baz"></div>
```

after this commit:

```html
<div class="foo bar baz"></div>
```

Another example:

```javascript
var buffer = new Ember.RenderBuffer('div')
buffer.addClass('foo')
buffer.pushOpeningTag();
buffer.pushClosingTag();
buffer.begin('div').addClass('bar');
buffer.pushOpeningTag();
buffer.pushClosingTag();
```

will produce:

```html
<div class="foo"></div><div class="foo bar"></div>
```

after this commit:

```html
<div class="foo"></div><div class="bar"></div>
```