Prevents inserting element into documentFragment

I was moving `if` and `unless` tests from `handlebars_test.js`
to be a part of `htmlbars` tests suite in this [PR](https://github.com/emberjs/ember.js/pull/9657)
and discovered a failing test.

This test failed with:

```
Failed to execute 'insertBefore' on 'Node': The node before which the new node is to be inserted is not a child of this node.
```

I **think** what ends up happening in this test is that inner most `{{view}}`
is being inserted in outer most `{{if}}` (well, not `{{if}}` itself but the document fragment).

I added a fix but I'm unsure if it's correct. @krisselden or @mixonic mind providing some feedback?